It certainly wouldn't hurt to drill a couple of holes and run in a couple of tapcon concrete anchors. If you leave them sticking out a bit they'll act as pieces of rebar linking everything together.

Try looking for concrete bonding primer, that's what you'll need to make sure it bonds well with the existing subfloor.

Forming can be done by anything that holds the concrete and doesn't absorb too much water. A couple pieces of wood will do fine most of the time for small jobs. If you have some melamine coated particle board which you can rip into strips it will work even better. You can use some cheap caulk to make sure the forms don't leak.

Normally you can walk on the concrete in about a day. However, it takes about a week before you can put a heavy load on it, and a full month before it's fully cured. If you can put flooring on it depends on the amount of moisture in the concrete. You can test this by putting a piece of clear plastic on the floor. If it isn't wet on the bottom after a day, it's dry enough.

No matter the isolation material, you need thickness for it to work when there is direct contact. Thicker insulation makes it work better, but no thickness at all (like foil) also makes it not work at all.

How much height can you spare?

First, give a good tug at those posts. If the holes are stripped out, there might be rot in those holes, making the post weak.

If it's still strong, then go with what ThisIsJustForPornog suggested.

And domed nuts, to make it safe for the kids.

If they would inform me about a mistake they made, they would get every next job too! Honesty should be payed back.

Now if they made the mistake and didn't tell me, then we're getting to your solution.