actionclaw

I'm trying to replace the starter on a 2004 Mazda 3, 2.3 liter, automatic.
I do not have a (Mazda, Chiltons,etc) shop manual. I found at least one suggestion online to address it from above after removing the air box, etc.
but, so far I've been approaching it only from below.

The previous owner removed and did not replace the plastic splash guard/kick plate causing corrosion underneath. This should be a super simple job but so far I haven't been able to get past the first nut.

I've applied penetrating oil more than a dozen times, used sockets, wrenches til the points got too rounded, then tried extractor sockets (resisting as long as I could but, eventually,) even resorting to vice grips but all to no avail.

Is there anything I could possibly be overlooking or is this just a particularly stubborn nut?

The only other thing that I can think of left to try is putting a torch to it which I'm a little reluctant to do. Could it make things even worse possibly causing irreparable damage the stud?

Plus, considering how difficult this one is, I have no reason to believe the second nut --that I won't be able to see and barely be able to access -- won't be as bad.

I should mention that though the old one looks horrible the only problem seems to be the wire that runs from solenoid to motor!
I try to do things correctly, already have the new starter and got it at a good price but I'm growing tempted to just try replacing that wire ..even if to drive it to a garage but then, once there, what would/could they do that I'm not?

Has anyone here experienced anything similar with replacing their starter? Any helpful insight, suggestions, etc are appreciated.

Thanks