Back when I built my army (like, literally assembled the minis), we didn't have a Death Guard codex. Everybody was just chaos space marines, and if you wanted to be nurgly (or khorny or whatever) you gave your guys Marks and Icons and did the same for your HQ, and that was that. Now, however, not only is that not how it works in the CSM codex, if you're running a Death Guard army you have a whole separate book.
This would be fine, but half my nurgle units aren't in the Death Guard book. I have nurgle bikers, nurgle raptors, nurgle chosen (with plasma guns everywhere), and nurgle devastators, to name a few. None of those units are in the DG book. So, I took a look at the CSM book (well, the wahapage thereof) and while it seems doable to have those units and give them the Mark of Nurgle, I'm left with a few questions. Specifically:
If I make a detachment with those non-DG guys, plus whatever I need to make it a legal detachment, and put it in an army with mostly DG detachments, how does this completely screw me? (I believe that mixing detachments completely screws one, according to el Internetto). If it does completely screw me, that seems a bit rubbish for anyone who didn't happen to build their old CSM army according to the future requirements of the DG book. My admittedly vague and half-arsed understanding of it is not only do the separate detachments not benefit from each others' rules (which makes sense) they actively lose those rules if you mix armies, which seems unnecessarily harsh.
Is there any known reason why these units aren't in the DG book? I mean, the minis exist. Why take em out?
Will this even matter in 6mos to a year, as I believe we're due a new edition sometime in 2023?
Cheers in advance all

Ps : I'm also still really annoyed that you're penalised for taking units in non-multiples of five. My DG all have 7-man units, because flavour. Now I either pay X PL for 5 dudes or 2X PL for 7 dudes. I have to leave 2 guys out of every unit, or pay way over the odds for a proper 7 man unit. Boooooo!