Any sort of EU federal state I would agree is nigh impossible - as it stands currently there is some lack of overlap on freedom of movement without border checks (the Schengen area) and currency adoption (all members except Denmark are obligated to adopt the Euro at some point in time, but Sweden at least doesn't seem to have any real intention of doing so) within EU member states.

On top of that, 'muh freedumb, they want to take away our right to self-govern' is used as a very efficient talking point for nationalist parties in all EU member states to shift public opinion against the EU (some possibly due to propaganda, others due to homegrown ideology). Any decision of importance within the EU has to be unanimous, so pretty sure there will be at least one state vetoing any motion to become a federal state.

Maybe once world peace is achieved, there can be an EU federal state, though even then I'd be skeptical.

For any brexit reversal, I understand the main issue would be that as a founding member, the UK had some perks (opt out of Schengen and the Euro) that would no longer be on the table for a new arrival. And there's the matter of breach of trust for already leaving once and causing major headaches.