Your sheet loads do increase if you travel out without easing the main. However it's pretty rare that you travel out this way. Normally when bearing away the mainsheet is the easiest to use to control power and it gets eased before the traveler is moved. In rare circumstances (death reaching), it pays to play the traveler before easing the main. It both reduces the angle of the sail and sheets it in harder (flattening/bending off the square head).

You are over-thinking...18squares used to employ such a track - but they are 11 feet wide and single handed (light) so they sheet in pretty hard when going downwind. A-cats used to use a similar setup for the same reasons before they went to curved tracks. For modern narrow/heavier boats, it's overkill to try and accommodate the slight angle change made by traveling out. You would lose as much time/distance while trying to make that adjustment than by just leaving it alone. You will normally have a little twist in the main when sailing downwind...so this really doesn't matter.

You will still need positive rotator for downwind tuning but you will no longer need the positive rotator for upwind use like you did with the boomless rig. You will need a rotation limiter, however, to keep the mast from over-rotating. Again, take a look at an F18 for a better reference.