You will need to get your boarding pass at the transfer desk in SIN. It should not be a problem at all. The staffs at SIN transfer desk are always friendly and helpful.

Regarding lack of response to the interlining baggage question, it is likely not many FTers have interlined between Garuda and United. All it takes is a phone call to United to find out.

I personally have only interlined between SQ and UA at SIN, and they have been seamless in each direction.