But now, in that scenario, the ‘injured’ player has to be substituted or taken off and can only come back on at next break in play.

Hopefully coaches/players will drill it into them that given there are now only 8 changes allowed that to waste one on a player ‘feigning’ injury will be criminal and even if said ‘injured’ player goes off to come back on at next stoppage, no team is going to want to defend a ‘short’ field for a full set of tackles with only 12 men

Hopefully this will see the end of the fake/cramp injuries at drop outs

It's not til there's a stoppage it's until his team regains possession or a stoppage or until the team gets an interchange card.

What the rule changes don't clarify for me is that if a team can't drop out in 30 seconds for an injured player is that a penalty under the posts?

No, for an injured player the shot clock doesn’t apply. The ‘penalty’ in those situations is you either lose a sub for the ‘injured’ player or he goes off, you keep your sub, but he has to go off and cannot come back on until next stoppage