See if this helps. I will try to answer one at a time for you for Delta. I am Plat on Delta and Gold on Cont to give you a starting point.

1) For both AA and DL, what would be the equivalent to CO Plat?
A- DL you need to have 100K for Plat, 50K for Gold. At Plat you get free access to the Crown club.

2) Which is easier to get comped to the equivalent elite level?
A- I have been trying to get a fellow employee who had 120K on Cont to Plat on DL. He had to send them a letter and a copy of his Cont statement to DL for consideration. He has not got an answer yet. When I talked to SMS desk they were optimistic about the comp.

3) The single most important benefit to me is availability of domestic upgrades to elites paying Y. How do AA and DL rank in that benefit? With CO its guaranteed unless sold out already.
A- With DL you have the ability to ask for an upgrade at the time of ticket purchase. There is no amount of advnce time frame. There is a problem with DL lowere fare classes (L) (U). These are not upgradable at all.

4) The second most important benefit is mileage accrual. CO Plats get normal mileage plus additional 125%. AA? DL?
A- DL you earn 100% plus a class of service depending on the level.

5) The third most important benefit (anal, I know) is availability of rewards - domestic coach, Europe business class, Hawaii business class/FC depending on config. Is there a clear winner between AA and DL here?
A- I can not help much on this as I have not taken a trip yet. I have looked for travel to Hawaii and had not found a problem getting flights for 4, being flexable on dates. As a Plat you have no blackout dates.

Hope this helps, I know I have gone through the same thing when I decided to go with Delta and I am happy with the service.

------------------