We installed a Sundance Cameo spa last summer using 6x6 pressure treated timbers with dovetailed very rigid and 7 inch lag bolts. Then it was leveled and filled with pea stone. We used 2 parallel and 6 across dovetailed together. The spa was perfectly level right outside our back door off a porch. No footers, installed per Sundance instructions. We drained and had it winterized while we were away for 6 weeks during the winter. When we refilled it during a 50 degree warm spell, we notice that it was no longer level either due to ground heaving or settling. It is off by a good 1-1 1/2 inches. All top jets are under the water and the water touches the 2 head rests on one side. We had no choice but to keep the water in the spa up to temp due to the temperature dropping below freezing.
We are hoping it levels back out when the ground thaws, but realize we are going to have to relevel it and possibly put it on 4 corner footers. In the meantime, how probable is it that there is a risk to damaging the spa?

Welcome to TFP!

As long as the tilt doesn't continue to increase you should be fine. The timbers should remain flat, even if not level, so there won't be any toque on the body of the spa and the extra water on one side of the spa should be well within it's ability to hold.

Do you think we need to put the 6x6 base on 4 corner footers or do you think if we just relevel it by sliding a 2x6 under the one side that it would hold level?

Adding corner footers is really the only proper long term solution. You can get by season to season by re-leveling each spring, ie using a 2x6 as you describe, but that gets old after two or three years.