Can't re install Mountain Lion

Question

Hi,
Long story short:
Have Mountain Lion installed on my early 09 iMac with 8 gigs ram
Filled up my orignial internal HD;
Had Mac Store put in a new internal HD
They must have installed Leopard or some eariler OS on it; when I got it home I was surprised that it wasn't blank; it booted as a new Mac.  It was otherwise empty. 
I copied my files over to the new internal drive.
When I tried to boot it of course it wouldn't.  I don't recall all the hassles I've had with it.
I wound up reinstalling from my Snow Leopard discs;
Now it wouldn't take my PW;
I used the terminal and made a new PW
Eventually I got to the SL desktop. 
Of course then I could not download Mountain Lion without first updating to 10.6.8 SL
So I did. And then it wouldn't let me log in again. 
I tried re making new pw's etc.,, etc., etc.  (can you tell I'm ****** by now?)
So i went back to re installing SL again; I'm back into my SL desktop (the version prior to the update). 
So now since Apple insists on using their Store app I cannot download my purchased Mountain Lion upgrade and i cannot get to a higher upgrade than the original SL version so I can't use the Store app. 
I didn't want to wipe the HD and start over; but is this my only option now?
Thanks,

Answer 1

When I tried to invoke Time Machine it said no useable B/U could be found, even though I'd been using it all along and was surprised there wasn't one. I don't have any other B/U system except my old internal HD with everthing on it--including ML OS. 
I'm not going to risk quitting the installer to try to save the huge ML file I'm still downloading; I'm going to just go for the automatic installation. I'd have to DL it again anyway even if I knew how to save a copy of it.  My understanding is it goes away once it's used--which is why I couldn't find it on my old internal disk. 
Update: as of a few minutes ago, after about the fourth attempt, I was able to update SL to allow me access to the Apple Store.  I did this by first making a new administrator account in SL that did not require ANY password. I reasoned that I would thus be able to gain entry no matter what; apparently it worked, or something else did this trip around the merry-go-round. 
Right now, I'm  stilll downloading the Mountain Lion update (glacier speed, of course). 
The final test will be if the whole thing blows up when I install ML.  If it does, I'm wiping the HD and starting completely over.  I'll then try to move my files folder by folder onto the new HD and go through the ordeal of reinstalling all my software again.
This is a true PAIN in the butt and time waster.  One would think that the updaters would be able to recognize and simply delete or quarantine incongruent system files so they can't wreck things.  And one would think there was a non-magical/secret way to download and make bootable a copy of any OS. 
Thanks for your reply.