hello there, i'm new to t25s myself, but i've been around vw's for enough years to put a dent in my kids inheritance
i have the same roof and the same hinges fitted to my van. here are my thoughts about what i would do if i were you:
first off, get the ripped/rusted hole in the roof repaired properly - if you can't do it yourself, and don't know anyone you could ask to do for beer tokens, get your yellow pages out and give a mobile welder a call
secondly - happy to be corrected here - i think the hinges we both have are either aftermarket or incorrectly fitted- if you look on the inside of the roof you will see some mouldings in the fibreglass - i think the hinges should attach to these and then to the roof so they are on the inside, not the outside - don't know if this is correct as i'm still trying to ascertain myself, but i'm fairly sure i'm right and i should have a definitive answer next week as i've managed to source a copy of the vw service workshop manual covering camping interiors and body repairs
re thickness of metal - most vw body panels use 20swg (1.2mm sheet steel)
re the fibreglass repair, if done okay, you will be able to put the hinge back in that position. you could run the risk of using self tapping screws, drill a 2-3mm pilot first and don't over tighten them though, but you're better off finding someone with a rivet gun and offering them a beer token; don't use bolts
out of interest, which fastening toggles do you have on the otherside? i'm guessing they are either loose or knackered. some on here suggest adding the fastening toggles to the inside of the van at the front and rear screens and above the b and c pillars on the inside - i'm still thinking about the merits of this one myself
i hope this helps you
cheers, billy
p.s. if you add your location to your avatar, you may find one of the kind folk from here may be able to help with the welding
p.p.s that's a devon roof, moonraker i think, not a westy - again happy to be corrected here