Precum more fertile??? That's a bunch of BS. Precum will never have as much sperm as ejaculate. Matter of fact, precum has very little sperm if any at all, since precum is produced by a separate gland located afer the prostate. Precum can only pick up any sperm located in the urethra. So if there is no sperm in the urethra, there will be none in the precum.
As to using both sides of a condom? I suppose you could but I don't know why you would even want to try. It's meant to roll on in one direction.
Pre-ejaculate is unlikely to have anywhere near the sperm concentration of ejaculate. However, that doesn't mean that it doesn't contain sperm. As a previous poster said if sperm is present when the pre-ejaculate is produced it will wash the sperm in the urethra out along with it.
As for using a condom on both sides...this is NOT a good idea. Once a side has come in contact with the penis it's possible that pre-ejaculate or semen has gotten on it. If you were to reverse it, the side that had come in contact with the penis would now be in contact with your vagina. If their was sperm on the side it would now be in your vagina. Likewise this would greatly diminish the condom's ability to protect you against STD's. Use a condom once...on one side only... and dispose of it.
Are clothes a barrier...unless the clothing is very flimsy or an open weave it probably is. That again, doesnt mean that semen/sperm couldnt find it's way through the clothing given time and sufficient fluid being released.