A few insights from my ICF build.
The planks can only partially bear as you have vertical rebar coming up from the wall below that needs to tie into the next wall.
You don't need to seal up the end of the slabs - a combination of the small diameter of the holes, interference from rebar, and by not working the concrete at that point (poker will vibrate the concrete at the bottom of the formwork), it simply won't flow in more than a short lenght if the consistency of the concrete is correct. You won't have an issue unless you are using an overly low slump / high flowing mix.
Also, you normally need to bend over a certain number of the vertical bars and break into the planks. Then get grouted in, in order to tie the floor to the wall.