One question, I just want to know if I understand it correctly:
If I will trade one Altcoin with low volume against another Altcoin with low volume, and there are most likely no other ASKs/BIDs on this pair (just in case ...), do I understand it correctly that these coins are traded in background against BTC/ETH with the Liquidity Aggregator to prevent my order stuck forever?
You got it right, and even better - STeX is trying to execute your order using all possible ways of exchange - against all other coins in the system as well. But for you it's just an ordinary order.