Adalah – as a condition in the imam of the congregational prayer and in the witness – is a level of fearing Almighty Allah that leads to avoiding the committing of major sins. Committing small sins without insistence does not affect it.
Adalah – as a condition in the marja – is a high level of God-fearing which leads the person to refraining from disobeying the religious laws and committing sins – whether the sins are small or major. If on the rare occasion he commits a sin, he would rush to repent to Almighty Allah and seek his forgiveness.
A person who holds 'adālah is called "adil".

Yes, it is obligatory to follow the most knowledgeable Mujtahid.

When a person starts emulating a marja after he realizes his responsibility in observing the rulings of the Sharia, he refers to the basic logic that every sane person will arrive at, and upon which one will be responsible to act in accordance to.
There is no doubt that this logic obligates the ignorant to refer to the knowledgeable in what the latter knows.
As the knowledgeable in the religious sciences (i.e. the mujtahids) differ in their views in various religious rulings, including their view in following the most knowledgeable, it becomes necessary to follow the edicts of one of them based on a distinguished characteristic that others do not possess, which is being the most knowledgeable.
Although the views of some of the mujtahids do not necessitate following the most knowledgeable, but in order to assure that we fulfil our responsibility of observing the Sharia with certainty, the initial logical ruling leads to following the edicts of the most knowledgeable in the issue of referring to the most knowledgeable mujtahid. This is so because the view of mujtahids who do not necessitate following the most knowledgeable allow following the most knowledgeable. To the contrary, the views of those who necessitate following the most knowledgeable do not allow following any mujtahid other than the most knowledgeable.
In other words, following the most knowledgeable is obligatory according to some and permissible according to others, whereas not following the most knowledgeable is permissible according to some and prohibited according to others.
It is natural then to follow the most knowledgeable in this issue. If his view – in this issue – necessitates following the most knowledgeable, one should follow him in the rest of his views. If his view does not necessitate following the most knowledgeable, then one can follow another mujtahid.

He should follow one of them because only one of them has the religious authority for the duty-bound person, even if he may not have initially identified him. The individual should determine this Marja in accordance with the rules of the Sharia, so that he may learn of the Islamic laws pertaining to the issues that he faces in the usual course of his life.

If one is aware that there are differences between the maraja's in the rulings that affect him, which is usually the case, then he must observe precaution. Since this is not practical for many to do so, he must find the most knowledgeable from them and follow him.

The scholars have mentioned in their treatises of practical Islamic laws the ways to determine the most knowledgeable, and one of those ways to is to refer to the people of expert (known as the ahl-ul-khibrah) who can differentiate between mujtahids and determine the most knowledgeable.

The ahl-ul-khibrah are those who can recognize the most knowledgeable by his writings, research and edicts. Those experts can be reached through communicating with the Howza. If such communication is not possible or determining the most knowledgeable is not possible, one is required to choose one of the scholars who have reached the level of ijtihad and fulfil the conditions of being a marja, and follow him. The chosen should be the most God-fearing amongst them if possible. Otherwise, the believer can choose one of them and follow his verdicts.

The emulation that Shias do is not blind taqleed. It is, rather, based on vision and wisdom, since everybody is asked to observe the Sharia and act upon its obligations and refrain from what it prohibits; and at the same time, a normal Muslim does not possess the specialized capabilities to know the laws of the religion. It becomes natural for those who are not experts in this field to refer to the jurist who specializes in deriving the rulings of the Sharia after exerting great efforts in order to reach religious conclusions.
Taqlīd is commonly practiced by people all the time, when they refer – in the aspects that require expert knowledge and scientific efforts – to those who specialize in the specific fields, like medical doctors and engineers, and they act upon their findings and conclusions.
Moreover, the Holy Quran and the Sunnah hold a central position in the process of derivation of the mujtahid, so naturally the rulings of a mujtahid – who fulfills the conditions of being a marja' – will not be against them.
Therefore, bearing all this mind, it is not within the non-expert Muslim's domain to judge a mujtahid's edicts, otherwise his judgment will be without any base.

It is not permissible to do taqleed of more than one marja.

It is not possible for non-expert Muslim to find out the most knowledgeable and God-fearing, he should refer to the experts to determine this. If you could not ascertain the most knowledgeable, or the most God-fearing, based on what the experts inform you, then you can choose one of marjas and follow his edicts.

One should refer to the trusted experts who possess the ability to determine the most knowledgeable in deriving the religious rulings and the most understanding of the religious texts.

If the duty-bound individual knows that there is a difference in the opinions of the mujtahids in the issues that he normally comes across in his life and needs to know their rulings – which is usually the case – then he should seek to follow the most knowledgeable amongst them and follow him.

Yes, as an obligatory precaution.

It is obligatory on individuals to follow the most knowledgeable when possible and that can be determined by referring to the experts who are able to determine who this is. It is not permissible for lay people to base the choice of their marja on learning about his verdicts and feeling comfortable with them.

A lay individual is not qualified to determine the most knowledgeable in the right way without referring to whoever can determine him amongst the experts, since they– and not any inexperienced person – are the ones who know the level of knowledge of the people that reached the level of ijtihad.

The said sciences are not directly related to reaching the level of ijtihād. However, during the research for a religious answer to an issue that relates to another field, like politics, economics or medicine, a scholar is expected to make sure that the issue is understood in such a way that the obligation before Almighty Allah to know the subject of the religious verdict is fulfilled. This might require that the religious scholar has some level of knowledge or expertise in these areas, or that he refers to the experts in these sciences. With this knowledge, the religious verdict is determined using the religious sciences and evidences that he already possesses.

Ijtihad is a talent gained after reaching a level of knowledge through which a student of religion is able to know the religious ruling from its rightful sources, i.e. the Holy Quran and the narrations of the Prophet and the Imams (peace be upon them). It is natural for this to require a long period of time and dedication, in addition to piety and God-fearing in such a way that one becomes sincere in his research for the truth, as it is prohibited to give an edict without knowledge, as the religious teachings emphasize.

It is compulsory to follow the most knowledgeable amongst those who have reached the level of ijtihad, and it is not sufficient to follow any mujtahid.

If the mujtahids are equal in knowledge or you are not able to determine the most knowledgeable amongst them, and if observing precaution – to guarantee that the religious obligations are fulfilled – is not a possibility, one must choose the most God-fearing amongst the most knowledgeable mujtahids. If this is not possible as well, any one of them can be chosen to be followed.

It is not permissible to depend on istikhara in determining one's marja.