It seems that in general college offenses are less efficient than NFL (with the caveat that so are the defenses) which means an "extra innings" approach would be less likely to result in a quick resolution to the game. Thoughts?

higher than NFL. Not sure that answers your question, but I would guess either the NFL or college football would resolve a winner in a fairly similar amount of time (and quantity of plays), with college perhaps being done even faster. I would add the must go for 2-points on a TD (less than 50% conversation) to expedite a final result quicker in the two TDs on first possession scenario.

the easiest one I can think of is to move field position from the 25 to midfield.

Is that it eliminates two very important variables from consideration: field position and the game clock. It also eliminates special teams from consideration as well, with the exception of the PAT/FG unit.

but with the playoff games taking over the sport, would 1 guaranteed possession under normal football rules extend the game a significant number of plays on average, with only 2-point tries on TDs, and if tied after 1 possession each, next score wins? Shootouts are tolerable in the regular season, but I would prefer the new NFL style OT in the playoffs. You have a better sense of just how big of an issue number of plays is for college age athletes, but unless that is a compelling case, use the NFL rules.

I admit to being a fogey and think tie games are OK The best OTs in sports are those which stay closest to normal rules. Baseball. Basketball, playoff hockey, and golf are good in that regard. Tennis and FB less so

Your idea of moving the ball back rewards god defenses. I hate OT field goals on an initial 4th and 15

Made OT special in playoffs

I agree with the rest of your post, too

At least have to make a 1st down or 2 to kick a FG

As a HS wrestler before reg season OT, I had 2 ties: 1 vs a very good wrestler. 1 vs a poor one
1 got me a big pat on the back & the other almost cost me my starting spot