How hard will it be to make a hive-enginesque diesel pool for the internal market?
A small fee, ~.1% to pay liquidity providers.
No staking period.

I'd say quite hard, there's a lot of things to create, the automated market maker, the liquidity token, the liquidity deposit, the liquidity withdrawal. It's definitely doable but it's a decently sized project.

What impact would this have on the speed of the base layer?

It would depend on the implementation, but I wouldn't expect it to be too craz.

Does hf26 appear to be on schedule?

No, blocktrades just increased the scope, see: @blocktrades/9th-update-of-2022-on-blocktrades-work-on-hive-software