I have just replaced the serpentine belt and tensioner pully on my 2007 Saturn Ion (160k miles). I haven't found a Youtube video with the specific problem I had, so I can't really reference anything, but I hope the below points are helpful for anyone who discovers themselves in my situation:
1) When the tensioner tool to take tension off is too long: There is a tool at Harbor Freight: 1/4 in, 3/8 in. Drive Long Reach Dual Flex Head Ratchet SKU #67994. It's long enough to relieve the tension and not too long so that it hits the frame. Harbor Freight has pretty good return policies.
2) Make sure that you use a screwdriver or something to push the 3/8 square drive pin deep into the tensioner, otherwise you might strip the tensioner, and create more trouble.
3) This was my biggest problem: The holes in the frame that are supposed to give access to the 15mm tensioner bolt were offset by about 1/2 inch from where they should have been. There was no way to get a socket head onto the bolt through the wheel well . At this point I should have just checked the tensioner to see that it gave good tension, spins quiet and didn't have any lateral play (It turned out that the old tensioner was fine, and didn't need replacing!). Instead I proceeded to get a flex head 15mm closed wratchet and worked the tensioner bolt off between the block of the engine and the frame. I spent 3 hours trying to work the tensioner, with bolt out of the car. BUT IT DOES COME OUT. THERE IS A WAY TO GET THAT BOLT AND TENSIONER OUT WITHOUT DAMANGING THE BLOCK, THE CRANK, ALTERNATOR, AC UNIT OR ANYTHING ELSE!!! Now, if the space you have available is 1mm less or more can make a big difference, but I got it to work for me, so it will likely work for lot of other poor unfortunate shade tree mechanics. I am not even going to attempt to describe how I got it out except to say that I maneuvered the tensioner till I could pull the bolt out through the holes in the frame with a magnet, and then worked the tensioner out through the bottom of the car. THIS IS PROBABLY AN EASY TWO PERSON JOB.
4) Getting the new tensioner back on took me about an hour. I found that if I used my fingers, up inside the wheel well between the frame and the block I could maneuver the tensioner to the hole and slide the bolt in and then maneuver tensioner and bolt to the right spot, and start the bolt into the threads. THIS IS PROBABLY AN EASY TWO PERSON JOB! You have to get the bolt in about 10mm before the same closed flex-wratchet will fit on it, and you can re-attach the tensioner. BE CAREFUL TO POSITION THE TENSIONER PROPERLY!!! There is a cylindrical post on the tensioner that fits into a cylindrical divot on the engine block. If the car is level then the tensioner pully will be pointing about 30 degrees down from level. Of course there's no way to get a torque wrench on the bolt so you have to do it by feel based on the force you needed to take the bolt off (manual says 33ft/lbs).

SHORT VERSION: IF THE HOLE IN THE FRAME DOESN'T LINE UP ON THE TENSIONER BOLT, DON'T CHANGE THE TENSIONER IF IT IS WORKING AND THE PULLY BEARING IS GOOD. IF YOU NEED TO CHANGE THE TENSIONER, IT IS STILL POSSIBLE TO DO SO WITHOUT DAMANGING ANYTHING, BUT IT IS TRICKY AND TIME CONSUMING.

 

Ugh, I now realize it is possible that the hole didn't allign with the tensioner bolt because the engine mounts are worn.

 

Was going to ask if you'd considered shifting the position of the engine.

Thank you for not erasing your first post. It serves as a wonderful illustration of troubleshooting yourself into something insanely more difficult than it should be. I'm not making fun of you; we've all done it.
The key is being able to look back objectively and figure out where you went astray.
You got the job done. That's what matters. Now you know what else to look for.

 

Nah, I'd already suffered through getting tensioner back on before I realized the problem. Now I gotta replace the mounts. Doing it AC Delco/RockAuto style will be ~$300! I wish I knew which non OEM parts makers were good!

 

I purchased an inexpensive aftermarket mount for my Grandson’s 2.2L Alero and it is still working fine. The factory oil filled one had let the engine settle about 3/4” and the crank pulley was rubbing on the subframe. I figure he will wreck it again before the cheap mount fails. LOL.