To break this down...
To get the benefit of REPS, it is a two part test.
1) more than 50% of hours AND over 750 hours in real property trades or businesses. spouses cannot combine time to meet these thresholds, one spouse must hit this on their own. If you meet this first test, you are a REP...now determine if you can use the benefits of it.
2) if you or your spouse meet test 1, THEN you can make an election to combine all your RENTAL activities together for purposes of determining if materially participate in your rental activities.
3) So you met test 1, made the election under #2 - now you see if you materially participate in your rental activities. The hours for material participation can come from both husband and wife. There are 7 material participation tests, the most common of which are at least 500 hours in the activity, or at least 100 which is more than anyone else.
You are correct that if one of you meets the REPS thresholds for 2024, and you make the said election, it does NOT free up any passive activity losses from prior years. This is actually a problem - since now you are treating them as one activity, they will carry forward indefinitely. If you dispose of one property, you haven't disposed of your whole rental activity, so that loss is not released. You can end up in an odd spot where these previously disallowed losses are disallowed nearly permanently until either a) you are no longer a real estate professional and you un-aggregate your rentals or b) every single one of your rental activities is disposed of.
I have one client right now with some material passive activity loss carryforwards, that the decision was made to wait to make the REPS related election until they utilize these loss carryforwards, so they don't become trapped. In our case they'll have it cleaned up within the next year or two, so the timing worked out well.
REPS status alone is not substantial a risk of being audited. It is literally 90% of my clients and I see maybe 1 audit a year at most, and it isn't even on the REPS issue. But ensure it is legit, if you are audited, and you didn't rightfully take the position, it's not just the tax - you'll be hit with late payment penalties, substantial underpayment penalties, interest, and potentially fraud related penalties if it can be determined that you knew you didn't meet the qualifications but claimed it anyways. If you did rightfully claim the position, the audit is just an annoyance.
Interestingly, each of my clients who have been audited who have had REPS status in place, it was never questioned at all... I can only assume that it is because the situation was so clear cut that they were a REP (IE full time development), the audits were focused around other issues.