My two cents:

1. Monsters is Leave Me Alone/Elvis and The Monster of Soul/ Soul Monster because a) the high-pitched/scream-y part fits Axl's description b) it has work from Marco Beltrami (on the other hand, there's nothing heard about Beltrami working on Monstrocity).

2. The General and Soul Monster/Monsters started out as separate tracks and were still separate by the time of Axl's chats in 2008, but were merged into one two-part song by 2010.

They are credited on all CD era songs released by the current lineup. There's obviously a deal in place.

However, it's not that different than the Appetite lineup getting credits for Hollywood Rose songs.

Writing credits for Perhaps and The General are on SESAC (which collects royalties for Axl). It looks that the full name of The General is "The General and Monsters"

Steve Freeman and Marc Haggard have written songs with Brain and Buckethead.

Sure, but like I said, he has done a bunch of other interviews and in none of them has he talked about GN'R. I think it's as simple that he was asked in this interview and and replied like this - maybe the actual question was as vague as the answer.

Duff has been doing a bunch of interviews recently and this is the only one in which he talked about GN'R in this way. And the "new GN'R album" bit is not even in the headline of the article, so it doesn't look like deliberate clickbait and it hasn't been picked up by any Blabbermouth type site either (which is telling about how much of a shit media gives about new GN'R music - the days that this would be big news seem to be long gone). But it's possible that he was paraphrased or misquoted (it's Daily Express, after all).

Most band members are capable of working on both GN'R and side/solo projects. Slash and Duff don't put too much thought into it. They write the songs, record them, release them. Duff said he wrote about 60 solo songs during Covid.