CYA level is personal preference. Can't really tell you that 30 or 40 is 'bad'. I personally shoot for about 50 in my pool, but that is what seems to work for me.

There are others who have a lot more experience in caring for pools (especially vinyl) than I do, but if you held a constant chlorine level over night, then I think you are good on the shocking, except for (maybe) the existance of whatever it is you seem on your seams. I'd keep brushing and vacuuming.

Poly is certainly an option to consider. Get the 60% stuff if you do.