After choosing a certain Marja who is qualified to be followed and following him, you should continue following him, until it is proven to you via religious evidence that indicates that there is a more knowledgeable living Mujtahid than the current Mujtahid. It is not permissible to pick verdicts from another Marja just because it suits you.
You should return what you took from them. If those items are not present, you should give something similar. If this is not possible, then the value of the stolen items should be given. They don’t need to know it’s from you, or that it is you returning what you had stolen.
It is a condition in the permissibility of consuming the eggs of the fish that they are taken from the water alive. If purchasing the spawn is not for the purpose of eating them, then it is permissible to do so even if the fish was not taken from the water alive.
Yes, one should decide which chapter to read before reciting the Bismillah. However, the overall choice that originates from repeatedly reciting a particular chapter is sufficient. For instance, if he is used to reciting Surah al-Ikhlas (chapter 112), then it is sufficient if he read it without fully being aware of this decision.