1 - It’s a moulded piece of silicone/acrylic that just covers the length of the receiver. Different manufacturers call them different things, but essentially it’s a receiver enclosure that still shows you where the receiver is, unlike a fully enclosed C-Shell type.
2 - Flat losses in (larger) canals are notorious for needing more bass. An 85dB receiver should comfortably cover it, but the option to go more powerful is there.
3 -Retention is down to a few things: Receiver wire length, dynamic TMJ function, the shape and slippiness of your canal, sweat, mould material, mould dimensions, how good the fit is etc. Some people just prefer the ease of domes too. It’s not a popular opinion on this board, but there’s loads of people walking around with losses like yours on a standard double power dome
Ask to try what works for you: if it doesn’t give a satisfactory fit/level of gain, get other options lined up.