The witnesses do control the interest on HBD in savings. So they could alter/stop that based upon market conditions.
Titanic crew could have altered the route at any point until they no longer could. But they could keep turning the helm afterwards.
I believe the haircut is hard coded into the blockchain.
It is.
Technically, if the community calls for a quick fix, consensus witnesses can collude (I stressed the consent of the Hive stakeholders - otherwise they'll get voted out for fraud) to adjust the HIVE price feed to allow or disallow further printing without a hardfork. It would be a hell of a mess to follow but not an armageddon (like pretending HIVE is worth more (turning the haircut off) lets people convert HIVE to HBD getting more HBD coins than the rate dictates for a dollar-worth-coin but everyone knows the HBD are worth less so it evens out). Unlikely scenario, but possible.