We were just recently on Serenade in an OS and are D+. SL offered comp drinks/snacks during Happy Hours 5-8pm. They didn't ask for our SeaPass card in there. We used our 5 drink pre-loaded vouchers any time in other venues.
No difference in the lounge hours SL vs CL/DL on Radiance class but you will have to use a voucher in the CL/DL if you opt to go into that lounge, and we see no reason to do that.
Vouchers are only given to C&A members based on your level. Booking a suite has nothing to do with those.....as you know.