I suspect none of the regulars here have experienced quite your situation, however here's some general information:

1. It's pretty rare for any airline to allow someone to voluntarily reroute themsleves ahead of time in anticipation of a bump. It might make logical sense in some regards, but it's generally not done. More commonly they'll say you need to travel on your original ticket or be treated like a stand by traveler.

2. Your alternate flights on Frontier are also sold out that morning. So that further makes the idea of trying to catch an acceptable early alternate flight DCA-MCI via Denver non-workable.

3. Your flight is sold out (as you're probably aware) but it may be nobody gets bumped if there are enough no-shows. And if more than 99 people show up, the odds are better than even that enough volunteers will give up their seats.

3. Although it should not necessarily be this way, a lack of seat assignment can put you on the top of the list if someone must be involuntarily bumped. So it's wise to get a seat assignment. If you have a seat assignment already, good. If not, getting one with online check-in 24 hours ahead of departure may be worth the effort. (Note that there may be seats blocked off, seats which you could not select ahead of time, which open up for selection at check in.)

4. Avoid selecting seats in row 11 if you can. That's because some configurations of the E190 do not have a row #11.

One last bit of advice. I suspect you're not all booked on the same itinerary, but if any of you are considered for involuntary bump, pointing out that you are all traveling together might be a successful last-ditch appeal. I doubt it will get to that, especially if you get seat assignments and get checked in.