I can't say I've ever boiled an extract but I don't think you'd see any benefits if you did, when I've used Coopers kits I just shoved them in the fermenter with cold water
If they have any solid gunk in them you'll probably be better with a brush of some sort. If not then a long soak with sanitizer will do fine. But I always rinse after use, otherwise you'll end up with mould and other unpleasant things the next time you use them.
The first time I brewed, I transferred as soon as fermentation stopped (about 7 days). On my next batch I gave it another week and I got better results, it was an identical kit