I want tp replace the foam side supports on my softside mattress. Will this work?

No! This is only a liner, and it arrives folded as flat as a pancake ... it has no structural support.

Will this liner work with straight wall style softside waterbeds?

It should. I used it when I converted a Sleep Number bed into a soft-side waterbed, and it worked fine. It doesn't stand up by itself like in the picture, but the bladder will hold the sides up.

Would this be considered a waterbed frame?

It is a waterbed frame for my soft side, but it has NO structure on its own. It is not hard. It is a shaped rectangular piece on vinyl that fits into the hole in my soft side waterbed.

What are the actual dimensions of this liner? The soft side water bed I own has interior dimensions of 65" x 69" and he 8" deep, will it fit?

I did not measure my liner before installing it -- but I remember being quite pleased that it was MORE than enough for my King Size Mattress, which is roughly 6'6 wide, 7' head-to-toe and 12-14" deep ... there was no need to worry about "if" it would stretch far enough, and it more than adequately wrapped around the soft sides. My only gripe recollection was the price -- but, functionally, it was fine.