What?!?!? Award travel/availability is the lowest of your concerns??? You could get kicked out of FT for saying that.

Seriously, it's good for you and for getting help here that you've made your priorities clear. By virtue of all of the travel you'll do, you'll easily get status that will get you extra legroom on any of the three US airlines. In fact, AA and UA might even have options for expediting higher status by a matching or challenge deals. More info on those can be found by searching those terms in the respective AA and UA threads.

Once you achieve sufficient status, that in turn qualifies you for lounge access on international trips regardless of what class you travel in. In fact, one very nice perk of the AA program (that UA and DL do not share) is that upon achieving Executive Platinum status (basically, 100K miles per year), for international travel you get access to AA and its partners' first class lounges even when traveling economy. For instance, AA partners CX and BA have some very nice F lounges in its HKG and LHR hubs (though CX regular lounges in particular are not too shabby at all).

It sounds like your company's flexible travel policies are such that you can save some money on some domestic trips that you can in turn apply to securing premium economy on international ones...?