The collateralization of tokens with HIVE is something I didn't expect. I assume it's for security, if something goes wrong with another coin's validator.

The overcollateralization is there to make sure there is no reliance on trust. All tokens brought in are protocol and economically secured so there is 0 incentive for hackers or bad faith actors.
Sure hackers could still potentially attack a single validator but that could never compromise the system or the liquidity in any way as they would lose in collateral more then they would gain in liquidity.

Validators secure the whole system, theyre not chain specific as it stands now.

Will they be rewarded with a dedicated token? Are there other use cases for that token to give it value?

There will be robust reward incentives included and a VSC token TGE coming in 2026. We did some marketcap modeling based on industry benchmarks and its looking good. Working on ideas for utility. A lot of things. So definitely keep an eye out for more information on that. :)