I actually only just started making my way through Year 24 works late last year, and only began reading Hagio's works a few months ago (she's quickly cemented herself as one of my all-time favorite authors), so this comment was a enlightening read, thank you for sharing!
Yeah, I mean I don't know for sure, but I'm pretty sure The Visitor was used to launch Petit Flower due to Thomas' continued popularity. Of course being Hagio, it's a great work and doesn't feel like a cash grab or anything cynical.
Petit Flower actually had a weird circulation format. It was quarterly from 1980-81, by the end of 81 it became bimonthly (odd months I believe,) then in 84 became monthly, then in 88 became bimonthly again until 2002 it re-launched as (Monthly) Flowers. (I admit I never get why sometimes these Japanese magazines relaunch as a new magazine that is basically exactly the same...)
You might be right that these formats were one reason that Mesh was structured how it was--like I said it echoed the format of Poe Clan which ran in the monthly Besatsu Shojo Comic (with around 50 page instalments as well) unlike, say, Heart of Thomas which ran weekly in (regular) Shojo Comic primarily in 13-15 pages instalments (if you read the collected version it's often obvious where each instalment would end, even if they don't mark it.)
I always wonder what it was like for the long suffering (in a good way

) readers of Kaze to Ki no Uta when it moved from weekly Shojo Comic to Petit Flower for its final 3-4 years who had to wait longer for each instalment, but I guess they got longer instalments... (And yes, all this stuff--how these stories were serialized, etc, fascinates me way way too much lol) Hagio of course
mostly stayed loyal to Petit Flower/Flowers and Shogakukan (where currently the Poe sequel is running) although there are plenty of one offs in other magazines and publishers, including her last long serial Queen Margot which ran in You from 2012-2020... Whereas Takemiya jumped around between publishers and magazines until her retirement to academia...
Do read Otherworld Barbara for sure--until I got to Hagio and Takemiya and other Year 24 mangaka I never would have said I much liked sci fi, but I love their stuff and Barbara is just... wow--if anything it tries to cover too much (from gender issues, to familial, in this case mostly father/son but also mother/daughter issues--so the usual Hagio themes--to cloning, space travel, dreams, etc) but is amazing even if after several re-reads I can't say I quite understand the ending. It doesn't matter lol
I think Barbara basically still has her post 1980 art style. It's been a bit jarring looking at the Poe sequel (which FG last year claimed they'd start releasing by the end of this year, but we'll see) with the massive change in art style. In general I find it fascinating that the 1970s shojo style was so... baroque? Like I said, with the lack of usually much of a panelling grid whatsoever, etc, and I wish I knew more about why there was such a change for all the year 24 group--certainly Hagio and Takemiya--in the 1980s although it might have to do with also writing primarily for an older audience (though the reasoning for this I'm not sure of

)
In regards to Lucien and Carnival--yes that's a great point. You could almost see it as, well, a young Greg--the start of the emotional breakdown of him. With Lucien there's still a sense of a struggle with his violent and harmful desires and maybe some attempt to control them, while by the time we get to Greg (who is basically as pure evil a character as you'll find in a Hagio work with not much nuance there) he's fully accepted that.
I'm jealous that you're just discovering the Year 24 group! And really, it's only been in the past 10-15 years that we've started to be able to read a lot more of their stuff (yes, mostly due to scans--which I'm thankful for--but also some published commercial works.) I'm old old old and discovered them through Hagio way back in 1995-96 as a teen when Viz briefly tried a Flowers line and released They Were 11 (and then the Four Shojo Stories anthology which also included Keiko Nishi slice of life stories and Changeling by the underappreciated "post Year 24" mangaka and a former Hagio assistant, Shio Sato) as well as Hagio's A Prime (aka A A.') a great series of three 1980s connected short sci fi works.
The release bombed of course (Rachel Thorn was leading it and I've gotten to talk to her a bit about it,) although around the same time Viz did release Banana Fish (initially hiding its shojo roots) and I thought that would be it for me getting to read any of this stuff, at least until I started getting (sorta) better at Japanese and found some French editions, so it does seem amazing that we have the stuff we have.