So on the topic of houses intermarrying, as has already been said above, it does happen with the noble houses, but there is also a mention (in the lexicanum article for escher) that part of the reason escher is okay with orlock is that orlock is known for political marriages, which would suggest outside the house.
For the shotgun, consider that van saar, the group that makes them and thus probably is getting a discount price, still pays more per suppression laser than the trading post charges for a shotgun, which would suggest that for outsiders it would be more expensive (potentially than even the combat shotgun).
In terms of the stc, I read the section in gang war 3 as more suggesting that Van Saar was in good standing with the mechanicum, so it might be more to the point to say that the mechanicum don't particularly care for the stc for whatever reason, they view van saar owning it as close enough to them owning it, or (as stated above) they plain just don't know about it.