Would adding code to my program to detect their absence/presence be a work around. If the files are not found, then disable program features as if the product was not licensed. Would that work?
No that wouldn't work, because there is the legitimate "first time" case where the files wouldn't be there. That's the point.
Also, I have one product, one version. Can I use the same product GUID and the same copy of TurboActivate.dat for product keys which are allowed 1 activation and product keys which can be activated more than once (e.g. site license keys). In other works, some customers will get a key that can be used once; other will get a key for 5 or 10 activations.
Yes.
What is the best practice to manage single licenses versus multiple licences for the same product?
Whichever you want to do. It's up to you. It's easier for IT administrators if you sell 1 key with multiple activations (because they don't have to juggle product keys).