Almost certain that they're too cheap to be tongued and grooved. If they're installed like mine were, you just need to remove the screws. The ones on the back could be quite long (2-3 inches), screwed to the studs.

Try "counter height office chair" on amazon. The chair base material needs to be stronger (i.e. more metal). The extra height would sway the chair a lot when you sit on it and crack the plastic base.

I like the kallax lines. Very "hackable".

You need to install sliding patio door lock. Need some work to install. You can also try sliding door ratchet lock. People usually use this to lock display cases at stores. But I don't think it'll fit.

Maybe try oscillating tool, assuming you have space there to work with one. If there's any Harbor Freight near you, you can get one around 20-30 USD.