Caveat emptor: IBKR's TOS makes it clear they reserve the right to liquidate your positions at any time for basically any reason, including margin requirement changes.
Every brokerage has those terms. Even Credit Suisse in this article had this right over Archegos's positions - they just were too scared to exercise it.
Of course. IBKR has lower rates and approves more easily, therefore they will be much more trigger happy. CS timidly emailed about potentially discussing margin rate adjustments, IBKR can liquidate your positions for instantaneous margin violations without warning or a margin call. The assumption is your leveraged position is hedged well enough to protect against untimely margin calls, and the cost of hedging is roughly equivalent to the nominal margin rate savings they offer.
portfolio margining does what I describe and has a regulatory minimum of $125,000 or so net liquidation value (portfolio size), SPAN margining can as well which is what the futures and futures options market uses at any portfolio size
so yes individuals can access both
the primary benefits are cross margining, using other assets to fulfill or calculate margin requirements for a new position
and portfolio margin requirements are easy to calculate, just take the loss at a 15% move and whatever that loss is becomes your current margin requirement, it is a 6% move for indexes (like S&P)
the way to get in trouble is by confusing the margining system and hiding exposure behind synthetic positions (example, a combination of derivatives to make it look like you are long stock)
On other hand Fidelity will reject portfolio margin even if you have significantly more than 100k.