The problem is unrelated to the flicker adjustment. What you did was damage your motherboard by forcing the shell together. I know because I did the same thing. Anyone else with unstable brightness at random, or when pressure is applied to the shell also has the same problem. It seems to be relatively common, I've read a quite a few people describing the same issue.
The 101 screen is thicker than the original, and needs to be installed with very thin double sided tape, in place of the stock adhesive dust seal, do not use the stock seal. The back of the battery compartment also needs to be shaved down a bit so that it doesn't butt up against the back of the LCD, so the shell can close completely, with minimal or no additional force.
After I forced it closed the first time around, and put the screws in, the LCD would go dim at random, as well as when I pushed lightly around a certain spot on the shell. Needless to say, I attempted to resolder some suspected components including the little transformer, tiny copper wire broke free, basically disappeared, and the board wouldn't turn on anymore.
Luckily I found a working glacier unit at the thrift store for $5, and gave it another shot using the precautions above. I took my time, and didn't screw the thing together until I had enough clearance for the shell to come together without any additional force. Very glad I did not sell the kit as I planned, because this is how GBA should be played. OG screen was a pain, SP is a baby's toy, DS doesn't feel right, and the Micro is just too small.
Before you buy from that highly annoying Rose Colored Gaming site, be aware that they are not immune to screwing up either. I have read at least one post from a customer, saying their unit arrived with the unstable brightness problem (I think somewhere in this thread). Find another donor and give it another shot, using the tips I laid out. There's more satisfaction in doing something yourself, plus you save money. As long as you avoid stress or bowing of the motherboard, you should avoid the same problems.
Adjusting the flicker is just a matter of knowing what to look for and how to detect it. Find a gray screen, glance at it and move your head around while making the adjustment, until you can't detect it anymore. Then stop, because it will only get worse from that point on.