HMAC(A, B) is just a more secure way of hashing compared to throwing a concatenated A + B into a hash. HMACs are used by all dice websites.

If you make 100 accounts, then a few of them might be unlucky. Some of them will be lucky as well, but many wins in a row aren't beneficial in martingale. Of course rolls have no memory, so an unlucky streak can happen to a player playing for the first time.

Every roll ever made is available on the website as a database dump for statistical analysis, something that only a handful of dice websites provide.