Well, they tried to do it until "First Class" (2011). "First Class" was designed and written to be a soft reboot. DOFP was an afterthought.
Absolutely ...... DOFP has got 2 versions of events in 1973, one version leads to the future we see at the beginning (and The Gifted seems to preced those events) ..... and then after the events of DFOP it leads to the future Logan wakes up in.
Neither version of events in 1973 that follow on from First Class leads to the original trilogy, and then the ages of Jean and Angel in particular in those two sets of films make them being the same universe impossible as well.
The fact it's the same actors playing the older characters need not come into it, It's the narrative we have to consider.
Legion is in a world where the government are only just learning about mutants which sets it apart from the reboot universe and I see it as part of the OT timeline for that reason.
The Wolverine, much as i dislike it and would happily write it off also seems to reference the events of The Last Stand with regards to the deaths of Jean and Charles so I'd sit that with the OT.
Logan seems to be in a similar world to the OT with a few changes but not too much to ignore if you don't try and make it fit with DOFP
The Deadpool films, until something says otherwise, I see as being in the reboot timeline, and for Wade to revisit the events of Origins:Wolverine with his time-hopping the only way I can see that happening and explaining the existence of another Wade in the past is if Deadpool's time-hopping created an alternate future from which came the pre-Deadpool Wade who worked alongside Logan, Sabretooth and Co, then after that created an alternate timeline Wade went back and stopped it by shooting alternate Wade.
As for "Quicksilver" and "Emma" in Origins:Wolverine lets just write them off as similar but not the same characters, it's not explicitly stated is it?
What that DP scene does do anyhow is remove the need to include Origins:Wolverine in the OT timeline and that works fine for me, and personally I wouldn't mind doing the same with The Wolverine but like I say there's arguments in favour of it being a follow on from The Last Stand.