Coated steel, including galvanized, welds better for me with my cheap non-gas Harbor F flux-wire welded. Probably because the nickel or zinc outgas to act as a shield (melts ~800 F). Try not to breath the fumes, though I don't think they are terrible like heavy metals, and indeed are in vitamins, but many welders say they get a headache after breathing zinc fumes. You would mostly be doing a bunch of tack-welds on sheet metal, to avoid burn-thru and warpage, so just hold your breath when tacking. Need to spray the backside before welding w/ "weld-thru primer". I just use Rustoleum High-zinc galvanized spray paint from Ace, and again after welding, grinding, and cleaning. I agree that using structural adhesive or even fiberglass matt can work. Those areas are of minimal concern re body structure. The lower rust is typical when leaf debris which got past the cowl vents collects. Water is supposed to drain out at the bulges in the plastic mud shields, but when I've taken them off my cars, that area was clogged with leaf gunk.

The corroded battery tray is typical. Can't buy a new tray affordably. I fixed one by welding on a 1/3 section of bent galvanized steel, but then got one from another car before using it. Didn't rust thru the firewall in my 2 300D's, but I caught it starting in the 1985 when there was a big nasty blob of white powder behind the battery on the firewall (zinc-oxide?). Chemistry can be evil.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans