What is the current timeline from application->approval for FLR(M) without priority service?
Same as it was for your last FLR(M) - about 6-8 weeks. I don't think the processing time has changed much at all in the last few years.
Since it seems like everything is online now, do I get to keep my passport during processing? Can I travel during the processing? Does my husband keep his passport?
You can now keep your passport, but I still don't think you can travel while your application is processing.
Your husband has always been able to keep his passport (for the last 2 visas as well) - it has never been a requirement for him to send it, just a copy.
What is the current cost? Do we have any idea of how much it might go up in the next year?
Current cost is probably the same as when you last applied, as it did not increase this year: €1033. Not sure if it will increase next year, or by how much though.
The IHS surcharge did increase from £500 to £1,000 though - not sure if it had increased before you last applied or not.
Can I use my income for the financial requirement?
Yes, you can. You can either use just your income, just his income, or combine both your incomes.
What happens if my application is rejected before my BRP expires?
If it is refused (not rejected) and your visa is still valid, you should have until the expiry date of your visa to either reapply, appeal, or leave the UK.
What happens if my application is rejected after my BRP expires?
If it is refused after your BRP expires, you will be given 14 days to either reapply, appeal, or leave the UK.
Having said that though, something like 95% of FLR(M) applications are approved, so if you meet the requirements, it's unlikely you would have anything to worry about. If you got your fiance visa and first FLR(M) without issue, there's no reason to think your second FLR(M) will be refused.