1. Discussion thus far seems to suggest it is medical and other technological connections braided. Sounds a reasonable guess.
2. Could be an attempt to visually represent them being "stretched full" by the drugs and hormones [steroids] used to Bulk Up. Actual side effects of steroid abuse, bear in mind, include baldness, acne, infections and scarring thereby, splayed teeth, tendon stress, and deep vein thrombosis. Or it could be a visual signal regarding their regularly being punched in the mouth.
3. Noble houses, yes, there is some indication in the fiction that they do, deliberately and carefully, to cement political arrangements. The Clan Houses? Well, there is that old scenario set regarding Romiet and Julio, two Juves from different Houses, and how they need to be stopped, stopped I say.
https://yaktribe.games/community/threads/romiet-and-julio.2086/
4. I would agree with those above who suggest that bolter rounds are simply easier to counterfeit reliably. That sounds a likely excuse to use.
5. Likely the Mechanicus would confiscate even a malfunctioning STC, were they actually aware of it. Bear in mind that we, the players, have a bit of an advantage over any character in the setting, with regards to omniscient world-view. Yes, we don't have one either [since information is contradictory and biased, deliberately], but ours is much closer to being such than anything in the setting. Except maybe Tzeentch itself.