LMDE is already on pipewire as far as I can tell. I have a process running by that name, as well as one called pipewire-pulse which I assume is providing some or all of the old pulseaudio functionality for whatever might be expecting it.
No problems I'm aware of. I thought I was having problems early last month, but that turned out to be hardware failure.
Sure, you still can customize the kernel, it's just not the same default kernel for LMDE. Kernels move differently in Debian but you can always install something like the Liquorix kernel if you need the newest, and Ubuntu still uses the HWE model IIRC.
What I mean is that it’s about the easiest package for the Mint team to package a custom version of. No dependencies to worry about. Or they can pull the HWE from Ubuntu and ship it with LMDE.
I don't think that's a good idea. Moreover, it would defeat the purpose of using Debian Stable as the base system and their magnificent team of kernel maintainers. If you want the HWE just use plain Linux Mint, if you need a current kernel, go with a rolling release distro, and if you need Debian, try Sid.