CJ, the floor shouldn't be dented, however there are a few reasons they do get dented. 1. Inconsistant mixes, ie not enough cement in certain areas, but whoever was trowling the 'mic' should have noticed it. 2. Soft spots under the 'mic', mud is notorious for this, any muddy soft spots should be dug out before applying the 'mic'. 3. Bad trowling, given the temp you describe, in a steel walled pool it's understandable that the 'human errors' will increase, but those should be patched before the liner goes in.
Given the temp, the floor should have been hard enough to walk on the next day. Talk to your builder and see what can be done about the dents (the only thing I can think to do is drain the pool, pull out a section of liner and have someone patch the bad spots - it'll be about 150 deg under that liner, a good incentive to do it right the next time, and it'll cost the builder some $)
Web, I don't like sand and cement because it's too hard, it can abraid the liner and won't let water pass through) Vermiculite gets very hard when it's fully cured (~27 days) especially with the weight of the water compressing it.
As Chardonnay (BTW welcome) -points out the liner has enough stretch that the dents won't hurt it, but for $1000, you should at least have a level area to walk on. - Waste