servos for a 35% extra 26 to 28 lbs.
For that size plane, a 4721 on each elevator should be fine. I would recommend using heavy gauge gold extensions, but a booster would not be required.
There may or may not be a benefit to dual conversion receivers. There are many variables to this, and it depends on the area you are in, the equipment in your plane, and several other factors. A dual conversion receiver filters the signal twice, which in certain cases helps avoid hits, and other cases doesn't.
Yes, you can run 2 packs through 1 receiver by plugging the second into an aux port. I would add a regulator to each pack to avoid any back feeding from one pack to the other.