Sandy,
The Lil' Miracle diaper has a large area to fasten the velcro strips to. If you go to their website, mylilmiracle.com, they have a picture you can blow up to see the velcro part of that diaper. The two strips can be replaced or resewn back in place if they get loose in wahings. This diaper is about the best diaper I've seen in quality of workmanship. The soft part of the velco is a big area, and it should not ever need any replacement.
Your other question on plastic pants. I will wear plastic pants over my disposable diaper on long trips in case of leaks, or when I don't trust the disposable to not leak. My plastic pants will only help in minor leaks. Before long with a major leak, you'll get too much urine between the two. But generally, I don't wear a pair of plastic pants over my disposable.
By the way Sandy, did your plastic pants come in you ordered from LL Medico? Weren't those the fancy ones?

...........Paul