I'd love to see the game designers include the improvements you mentioned into the default UI. In the meantime we have added some nice features to
DarqUI Premium that make it easier to deal with all those mercs.
Mercenary Favorites (choose a short list of up to 10 mercs to appear on a separate tab)
Battalion seasons display
A tooltip that reminds you which mercenary is currently in training
In addition to One-Button Pet Pull, Shrink Mercenary shortcut and salary reminder features found in standard DarqUI, the premium mercenary window has some nice extra features (all optional) including an automatic delayed assist control, a "hybrid" layout option that does not minimize/maximize, and you can summon and dismiss your current merc directly from the merc window without opening the Character window.