How many days at a time does a reserve sequence normally entail?
This is variable from fleet to fleet. The number of days in the sequences tend to mirror the trips in each fleet. June is a 28 day month and reserve lines have 15 R-days.
777 has a fairly even split between R schedules with all 15 days in a row and two blocks of 8/7 or 9/6. Minimal short trips and single day hub turns, so no need for smaller sequences.

Airbus has slightly less than half their R-lines in one block of 15. This is to cover a pretty large number of 12-ish day trips where crews deadhead to Europe, operate there for the trip and deadhead back.

The rest of the Bus R-lines are 2 and 3 blocks of days with the smallest sequence being 4 days. The rest of the fleets lean more towards 2 or 3 blocks of days but still have some lines with 15 in a row. No sequence is shorter than 4 days.

5 week (35 days months) have a similar pattern. One block of 19 days in a row as well as two, three or four blocks of days. Min days per sequence is 4 again.

How much difference in last 5 FAE does sitting RSV CA make over being more senior WB FO? Does the latter have the flexibility to render the difference modest while preserving QOL?
I think an FAE is a contract? But I get the gist of your question. A 777 Captain at max pay that rarely flies making reserve guarantee each month makes about $60K/year more than a middle line holding FO also on max pay for his seat. Ruling out the Captain might be on reserve at home in MEM living the dream, assume both guys commute. The Captain is going to "work" (i.e. be away from home, on reserve in a crashpad) an average of 2-3 days more a month than the FO. So, roughly 30 more days of work per year. When you do the math on work days vs pay for each guy, you end up with a "per day" rate for each pilot that's within $100 of each other. If the Captain on reserve flies all the time, what I'm saying is N/A. But 777 Captains (or FOs) on reserve don't fly all the time, by a long shot.

777 FOs in the top half of the seniority list can hold carry-over trips which can allow them to fly extra each month at straight pay if they choose. Other FOs without carry over can pick up extra trips if they have makeup time available or on rare occasions fly draft (overtime). An extra 3-day trip each month will make up the difference in pay for the year. So, absolutely yes, the senior FOs have the flexibility to make up the difference.

How does the "commuter protection" work?
If you miss a trip while commuting on a company jumpseat, the trip is dropped without pay and you will not be subject to discipline, provided:
Elapsed time from the showtime for your jumpseat to 30 minutes after final block in for your first duty period does not exceed 13:30. Additionally, you must have 1:30 from scheduled jumpseat arrival to show time for your trip.

These parameters are usually not difficult to meet if you're based out of MEM or IND and flying domestic hub turns. The whole policy is N/A for anyone commuting to an international trip or ANC just based on the requirements.

I've used it once in 12 years due to a divert and it worked as advertised.

Saw the bit about 400 DEN commuters - how often are all seats full?
From what I've heard, more often than not. Especially at the start of the week and end. Tough commute.