If the carpets donated as religious endowment (waqf) were many in quantity, such that the mosque does not require them all and are therefore not used, it is permissible to transfer it to another mosque. Otherwise it is not permissible.
You can reply to the greetings of 'salam' by just saying 'Wa Alaykum Salam.'
It is permissible to collect the mentioned interest, but you should not intend the interest-based deal. However, any funds received from the bank should be taken on behalf of his Eminence and then one should take ownership of it after that. It is better to help the needy believers with some of the collected interest if all of it is not needed.
The Shirk belief that takes the person outside Islam is believing in a partner to the Almighty Creator in His creation, like believing in a god for goodness and another for evil etc. Another form of it is worshiping idols as Arabs used to do in the time of ignorance before Islam. Lesser Shirk is, for example, showing off in one’s acts of worship. This is a major sin, but does not take a person outside the fold of Islam.