Before I start coding it from scraps, is there anything inside vanilla/forge that would allow me to deactivate all player actions or at least part of them?

 

Goal:

When player joins game for the 1st time (or his data is corrupted) he receives packet that opens Gui (which cannot be closed until all tasks are finished). During this time I need player to be literally non-existent (can't be moved, attacked, pushed, fell, etc.).

LivingUpdateEvent - check for the player, then cancel the event: the player entity will then not process entity updates, so can't move, be attacked, etc. You'll have to test if it works for your specific case, as canceling this event has a very wide-reaching impact.