No way you could have known really... maybe a hose to check for leaks, but overall you aren't doing too bad. Persectives are, look at what it would take to fix it properly, and look at what you are proposing. Youre looking for the easy way out, and tell me how often that is the right thing to do??
I think either way you will need to remove the dash to do it right. So why not patch the rusted areas by welding? There wont be a time better than now to dig into that...
I somehow figured out how to weld with just a few times of practicing & it was much easier than i ever imagined. The biggest hurdle was just overcoming fear of not knowing if i could do it or not, but once I bought all the equipment, it was easy. I practiced on old junkyard fenders, and after an hour, I knew enough to start fixing my rust issues. It is kind of fun, & satisfying feeling knowing how solid it is with metal welded together, knowing its gonna be good for decades vs bonding or epoxy that just is not gonna be as stout a repair. I vote for you buying a welder!! It is my duty to push you over that edge!! If I can do it, having never welded before, so can you!!