Tonyf, I believe your fears of a portfolio wipe-out are unfounded as we have filters that should prevent large movements in stocks and the executions are done after the open. But if you are really shorting stocks that are HTB with very low volatility, you may want to rethink your strategy or perhaps look into hedging strategies if the potential for a blow out is probable.
I followed up with our US office to confirm the US process. The buys are not pre-open or MOO.
We notify clients there may be a closeout (reg 204.
https://ibkr.info/article/3239) after 3:30 PM EDT and then again after the market close. The actual buy order is submitted at the open but depending on size it could be submitted as a VWAP or perhaps another algo.
Note: the team does resolve the vast majority of issues and if you have hard to borrow shorts with reasonable size, contact the SLB customer service desk who will do their best to provide some color.