Clean the spray head too ocasionally, that helps and make sure the vent hole is clear. And if the glue stays in the gun a long time it can get thick, you can pour it back into the glue can , shake it up well and it should be ok, if it's real thick pour it out and clean the gun thoroughly.
I would NEVER use spray can glue for a headliner, it will fail plus it's more expensive than using Weldwood in the long run

You can buy weights for that purpose, we have also used old florescent light ballasts

I don't know what they are using but we have redone several MBZ vans where the seats and armrests were peeling and delaminating. And as far as UltraLeather, I had to redo the seats and wall panels in an airplane that was done in UL, it was terrible as bad as the stuff in the vans.

I worked out of a GMC 14 foot stepvan for about 16 years, had my sewing machine mounted and a toolbox with wood on top for breakdown and layout, air compressor etc..
I currently have a Transit 150 van, it's big enough to do mobile sewing but I don't do that, I have a shop too. It's hard to know what to carry on the van these days with so many different vinyls leather and cloths.

On some of the upholstery forums guys have mentioned sprinkling baby powder or cornstarch on the board , they say it absorbs the oil from the deteriorated foam, then brushing and blowing it off. I haven't tried that yet. We have used full strenth Castrol Super Clean ( the purple stuff) you do have to let the board dry well but it works ,after it dries the board is not tacky or sticky.

I think once it has gotten to that point nothing is going to soften or restore the leather enough to be pliable so that it can be resewn. I’ve tried a lot of things but none really worked.

Around our area some stereo shops and other places are really cheap selling and installing afermarket leather kits, and as has been mentioned, they often do not fit as well as factory covers. Almost every kit that we have done recently we had to modify some parts to make them fit.
We don't do many for that reason, we have plenty of other work that we can make good money doing.

We have worked on several newer Lincoln seats, they are pretty easy to do, the backrest covers clip into the seat . Poorly designed IMO and as with other manufacturers they use vinyl for the bolsters and it tears easily at the seams, it will mean a lot of work down the road for trimmers.

Simple Green diluted about 2 ounces to a quart of water is a pretty safe cleaner for most anything inside of a vehicle.

Those are great, I got something similar from a can company here locally years ago, cores for rolls of aluminum. Great deal for anyone in your area!

Some of the Mercedes late model seat covers (E350) are not too expensive from the dealer but I get the solid and perf vinyl from Coast to Coast and just remake the whole cover (reuse the attachment and rear cloth panels) not too hard to do and I make pretty good money on them. I do usually glue some thin foam to the vinyl. The Coast to Coast vinyl seems to be a much better material than the factory MBZ vinyl too!

I got some years ago from a place that makes aluminum cans the tube were cores for rolls of aluminum. I had to glue 2 together to get them long enough. Yoy might check on places that make cans in your area.

What is MAS backing?

Rubbing alcohol or 3M Adhesive Remover seems to work well , but I've also cleaned it off w a magic eraser pad and some Simple Green.
I try not to get the lines on any part of the seat cover that will be visible after sewing it.

I find the fit of the ACC carpets to be inconsistent, some of their molded carpets fit pretty good, but others are just terrible, and on cars and trucks that have been out for many years, you'd think they could make a mold that would fit, the factory carpets fit!
We don't do a lot of retail carpets but when we do we will often just price the factory carpet and upsell it to the customer, as those are perfect in color match, precut holes and they FIT!!!! and are fast and easy to install. Unlike some of the ACC carpets.
Ordering w/o the padding is a good idea though.

8" thick foam is probably too much IMO you might want to measure the thickness of a standard bench seat and the back is generally less thick than the bottom.
Plywood support will work but make sure to use some good dense foam.

Sounds like a LOT of work, why not just buy some perforated vinyl and use it?

If you have a steamer you could try that too, steam works miracles on wrinkles with fabric and vinyl, not good for leather though!

You can get vinyl that is made to look like alligator, and other exotic textures and also cow leather that is embossed with gator and other grains.
Real alligator hides would be hard to work with IMO and expensive and not consistent in thickness etc....

Most suppliers like Albrights carry the embossed vinyls.
A quick web search found this place with embossed leather hides, and I'm sure there are a lot of other places to get it
https://www.leatherhidestore.com/embossed-leather-croc-embossed.html

I've seen some LED magnetic lights for sewing machines at South Star Supply and on eBay, thinking I will purchase one soon to try it out.

IMO there are too many variables, there are so many different types of seats , headliners etc...
Some are fairly easy, others are very complex.
Even if we had something for interiors like the estimated time listed for installing convertible tops, the skill set of the individual trimmer will vary so the time estimates will not be accurate.

Are you talking about the door inserts on the newer VW Beetles? those are a real pain to recover.
That residue is like headliner foam that has disintegrated.
I have used 3M Adhesive Remover, Acetone / Alcohol mix, Laquer Thinner, it's hard to totally clean that off.
FWIW Coverlay makes a hard plastic panel that can be dyed and installed to fix those VW door panels.
They fit and look pretty good and are pretty easy to install.

There are a lot of unknown variables , it depends on what kind of seat, some are fairly easy others are very complex.
Same goes for headliners, steering wheels, headrests, armrests.
With time and lots of practice you will get faster and be able to more accurately estimate how long a job will take.
Even if we had a book with estimated hours for specific jobs, it could still vary depending on the skill of the trimmer.
Convertible top books often have estimated installation times but there are still variables, unforeseen problems etc...

My late father in law made me a shelf with dowel rods about 20 years ago, I'm still using it. Thread on top, I store chalk, razor blades etc on the lower shelf.
I have a sewing area room that is separate from other parts of the shop so dust isn't really any issue for me. And mine is just flat, though the angled unit would work well too it seems.

Sometimes you will get lucky and the glass guy will get the windshield out without breaking it, I repaired a shrunken leather dash on a Porsche recently and they got it out w/o breaking it.
But there is always a chance that the windshield will break in the process.

I have not done a Camaro headliner, but we have successfully been able to get the headliner boards out in some other vehicles where the windshield was supposed to be pulled in order to get the board out. And there are some where we just had to have the glass pulled.
On some of them I think they build the vehicle around the headliner!