Three other thoughts:

MCI used to be a hub for US, so consider those options when evaluating UA, its *A and codeshare partner. Going from MCI to FL, CLT would seem to be an obvious stop, for example.

(The next two may not meet your 2 & 3 preferences, but . . .)

Midwest has a fair number of direct destinations from MCI (including LGA and DCA). I believe they partner with AA, so it seems like you should be able to receive AA miles for the Midwest flights. I haven't heard much about them lately; I don't know what kind of financial shape they are in or the extent that they match the lowest fares of others in the market. No F but their traditional Y product used to be better than most and competed with other's F product (I think that the traditional Y is now their Signature Series).

If you travel west a lot and would have to connect anyway, also look at Frontier, who is also a partner of Midwest (but not of AA or UA). They do fly direct to DEN and LAX from MCI. No F that I am aware of.