I never really liked the whole sabotage is just "posturing" opinion.

Like there's real physical stuff destroyed (or in most circumstances digital stuff). How hard is it to impound ships that break stuff and etc so that the ones responsible are actually punished?

Probably no harder than impounding illegal unsafe unregistered oil shipping transports making their way through the Baltic->Black sea right now, evading sanctions.

Not hard. Not done. Because we're cowards.

Didn't the cable get cut before the announcement regarding Ukraine being able to use U.S. missiles against Russia?

Possibly, but only by hours—the news about the missile announcement was on Sunday afternoon, and the cable was cut Sunday morning. But it's unclear when the policy was actually communicated to Ukraine and Russia almost certainly found out about it immediately after Ukraine did (or even before).