Looks like Pa is as brutal as this state, and imports seem especially good at lightening themselfs up.
Yeah, a lot of newer cars, including domestics use awefull thin metal. I've worked on a few newer cavaliers, and the metal is so thin, you can literally dent them with thumb pressure, including pushing some dents out by only getting behind it and pressing with your fingers. Least its on the quarter, really large flat areas like the tops of decklids and hoods can get real hairy welding up without it caving in, or chasing warp around.
Do you have a welder? If you do, you can still probably weld, but must weld in a short sections of you patch at a time and skip around where you placing the short weld sections. Hammer on the welds with a hammer on dolly as you weld it up if possible ( May be difficult in that area unless you can get a dolly behind through the rusted out wheelhouse or have acess inside the trunk and a helper) (the heat from welding will want to shrink around the weld area, and a hammer on dolly will stretch. Keep a air blow gun nearby to keep it cool as you place your welds. You still may need to do some hammer and dolly work or use a stud welder to straighten things out as well as you can after welding in your patch, so you aren't using a ton of body filler over it. Once welded and welds ground down, I like to epoxy prime first, use a thin coat of fiberglass filler over the weld area, and then finish blending in the rest with plastic filler. The trick is to put in as little warpage as possible while your welding in the patch. If your patch is thicker gauge then the metal on the car,angle the weld nozzle to concentrate most weld and heat on patch to avoid blowing away the thinner stuff your welding to.
Unfortunately I really doubt you will find anybody selling premade patch panels for that car, So you would have to make some. If you just fiberglassed and bondo'd it up, it would be rusting and the new paint bubbling up in no time.
I'd probably pick up some twenty guage sheetmetal, even if what they used on that car is thinner gauge, 20 gauge is fairly easy to form by hand. Or maybe you could luck out and find an old fender in a bodyshops junk pile that has a wheel well lip that will match up that will be able to use as a patch, may not hurt to just stop by one and ask, maybe you will luck out and find a nice shop owner that will have something that will work decent in his scrap pile and let you take it. Or if luck is really on your side, find a quarter panel from the same car that was replaced for damage in another area, but wouldn't count on that. You could also check with some junk yards but they may just be in the same condition.
It helps to make a cardboard pattern (as well as look at the other side, if its still in one piece) to figure out how to make your patch, and you can leave enough excess to allow yourself to make a folded over edge along the bottom like the rest( above your bumper cover). First put an any little curvature if needed to match the quarter (i put pressure on the edges of the patch, and put some curvature in using my thigh or knee as backup)in your patch piece, Check by placing it on the quarter behind where you will be patching. Then You can cut some slits the length of the fold you need to make, which will make it easier to form over nice and crisp (and hammer the tabs over a sharp edge of a table or something, or use a duck bill vice grips), as well as to form where the body line-wheel well lip will be, and cut off the excess metal on your patch where the bottom lip and wheel wheel lip. Then when you have a patch that looks like you made somthing right to replace the piece, weld all the slits and areas back up on the patch.
Start Welding it in by tacking along the top, and then down the sides, keeping tight to the quarter panel with clamps or pressure from a hammer handle as you tack in. Once tacked all around into place, then weld up completely placing short welds and skipping around.
As far as the wheelhouse and inner quarter, figuring you don't want to replace the thing, maybe clean up inside well with a sandblaster when you cut out your quarter, and glue in a patch, epoxy and blend in where you will see some with fiberglass filler and then undercoat it. Or por 15 everything inside there and the wheelhouse. You could also glue in your patch for the quarter as well, but then being as the whole piece isn't replaced, its possible a ghost line could show up eventually in the paint, around the patch. But as far as the patch panel adhesive itself, the proper stuff is strong and can be used for non structural panel repairs, means no heat or warping panels or burining off of corrosion protection, and the adhesive itself will provide good protection of the lapped seam area. Don't forget to apply corrosion protection to the back of your patch (do it before welding in and grind off where needed if lapping and spray on weld through primer, unless you will have access afterwards to treat)