Given that they've already provided an "equivalent item level" for monsters (i.e. +1 at 1st-5th, +2 at 6th-10th, etc)... this being the amount of plusses from something that they shouldn't get because it's assumed to already be calculated in... then I personally would be inclined to give monsters in general +XdY crit damage, just as if they were/held a magic item of their assumed threshold. (X is that threshold, so X = 1 for a 1st to 5th level monster.) For the die size, Y, I would probably use d6es for the first three levels of the range, d8s for the latter two in a given range. (Yes, this becomes discontinuous at high levels - 5d8 at level 25 being higher on average than 6d6 at 26th. The algorithm could easily be smoothed or you could just ignore this.)
If I were worried about this, anyway. Currently it's below my "this is important enough to actually houserule" threshold. But we're only 4th-5th right now, this could change. And I can see how a leader-heavy party would want to negate the "you bounce straight to 0" effect.
Just as a thought - you might consider reducing the effectiveness of that rule, if your players are looking for more challenge. Say, for example, that healing someone below 0HP doesn't instantly bring them back to 1HP and then some; it merely has double its normal effect, to a limit of no more than (the non-doubled healing amount) above zero. So if I were at -50 and got healed for 15, I'd only go back up to -20, not to 15. (You should probably allow this to also stabilize them automatically, at least at first.) But if I were at -10 and got healed for 15, I could heal all the way to +15, at which point the healing is capped. This keeps it strictly less powerful than the original rule in all cases, and often much less powerful.
Then being downed would be a much bigger deal for your players, without having to resort to coups de grace. And would make the auto-crit from a CDG much scarier, since it'll take a lot more to prevent the monster from finishing the stroke on its next turn. (Which you can then narrate as a single attack which killed him, and the monster spent its second action just making really sure he was dead - akin to the next three bullets after the fatal one, in many an action movie. Or simply taking a little snack before returning to the fight, depending on the monster. Or reskin it in the other order, and the first "coup de grace" gets retroactively converted to "it was leisurely getting your head into an optimal position between its teeth" and the like, with the second being the single blow that kills.)