Answer
(Fatwa: 1912/D=396/k=1431)
(1) It is not wajib to refrain from eating and drinking at someone’s place until one is not sure of the unlawfulness of one’s total income. So, when they have the business of jewelleries etc then on their request it is allowed to drink juice etc. If someone avoids for for some reason, it is a different matter.
(2) It is allowable to take gift etc from a non-Muslim. Also, it is permissible to eat and drink in the case of mixed income. Yes, if someone wants to avoid, it is better for him.
(3) The loan that they have taken from the bank is not interest; hence its ruling is same as answer 1 and 2.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband