Did the screwdriver slip? Did dust fall off onto the board? Lint from your shirt? Did you tug at any cables?
Did your shirt brush on the board as you were removing screws and whatnot?
All these little things matter.
On the restocking fee, it's there because they can't sell it as new. This is why I sell the service, not the product, selling the product would be hell. It's worth little used. Would you have bought it used for over $140? Few will, and the restocking fee is there to reflect that no one wants used screens.
Also, the LP154WP2-TLA* and B154PW04 V.6 are not only compatible with each other, but also of equal quality. I kind of prefer the B154PW04. They are getting increasingly difficult to find. So if you want to pay a decent price to buy one, most will delegate to whatever is available. Be glad you didn't get sent one of the old Samsung pieces of crap.
That is ebay. buying one at a time at a fair price, it is very hard to avoid getting a complete knockoff altogether, at least they sent you an original model.
To fix the OS problem, hold apple S right as it turns on. Wait for the black boot screen to come up.
Type fsck -fy and let it run. You should be set to boot now.
As for the backlight, that's your adventure. A few hints:
a) The left side I/O board powers the inverter which powers the backlight. Look here for a problem. Messed up board traces, etc. A conductive pen helps with rewriting traces.
b) Sometimes the inverter dies. Buy a new one, it's $5.
c) Sometimes the board is loaded with pieces of grit, dust, and gunk. a lot of it you can't see. Running it through an ultrasonic cleaner with distilled water and an ammonia based cleaning agent lacking cations/anions(read: NOT AMMONIA, AMMONIA BASED) often fixes these issues.
If the old screen doesn't have a working backlight either, it is highly unlikely a new display panel is going to fix anything, since there's now a problem within the machine itself.