If Custer had ridden to support Reno, wouldn't that have had the added advantage of blocking the Indians southern escape route and driving them North into the planned entrapment?
Yes, if Custer had crossed the river with Reno it would have better sealed off any escape to the south. It also would have had a number of other advantages.
It would have been a safer attack because it would have kept the two battalions in supporting distance of each other and allowed them to better defend themselves if the battle turned against them. It would have avoided the high risk associated with splitting their attack against a superior enemy.
It may also have increased their chances of victory. The soldiers best scenario was to catch the village by surprise and strike with maximum force when the braves were unprepared, the women and children were in the village, and the pony herd was grazing at a distance. Taken by surprise, even the most disciplined fighters will often break and run.
Against all expectations, they did take the village by surprise but Reno's battalion was too weak to deliver the blow needed. With Custer taking part in the charge, I don't think Reno would have stopped short of the village. Instead of 140 men stopping and firing at a distance, they could have had more than 350 charging into the village itself.
After that, things get increasingly speculative. In the short run, I think the soldiers could have occupied at least part of the village. Maybe that would have been enough to cause the Indians to withdraw from the fight, but I doubt it. Likely, they would have rallied and made it difficult for Custer's men to hold their gains.
But even if the soldiers were forced on the defensive, I believe they were far better off fighting on the flat land by the river instead of the bluffs on the east side, where Custer ended up. The open space would have allowed them to take advantage of their more powerful rifles. If necessary, they could have retreated to the cover of the trees along the river, where they would have had a strong defensive position, and where Benteen could have joined them later, along with the pack train. Maybe most importantly, they would have had access to water if they did end up besieged.