Can I do this? (split classes of service)
 

We fly out of our regional airport and it's almost always a connection through ATL. The flight is under 30 minutes so there is never a need to pay for anything but main cabin. But when we connect and have a 3+ hour flight segment then we like to go Comfort + for the obvious reasons.

I've not tried this but can I split the flight segments into main cabin for the one segment and C+ for the other?

You might be offered the ability to upgrade individual segments online post-purchase. No way to do it at purchase time without creating a (likely very expensive) fare break due to C+ being a separate fare class. Exit row seats have as much (or more) legroom as C+ and can be bought independently.

As far as I know, exit rows are reserved for elites only, as preferred seats, and cannot be paid for. (Perhaps at the gate?)

Preferred seats are available for purchase by non-elites starting at 45 days prior to departure --

https://pro.delta.com/content/agency...d-seating.html