Actually crushing blows are not really that big a deal unless you're in danger of being 3shotted, which is unlikely from a well geared 19 tank, most have more health than a typically geared level appropriate tank.
The issue is threat generation from a tank who can't hit the boss but is taking huge damage. I actually used to refuse to tank instances that had bosses 4+ levels higher than me back when I was levelling my first warrior, simply because I could do without the headache of dealing with people who had no idea what that would do to my aggro generation going all out and then screaming 'wtf spam teh tont button!!' When I did do it however, there was never any issue with healing, although my druid buddy was a pretty good healer.
Multiple healers would help as it spreads out the heal aggro. Gearing/speccing for aggro gain primarily and mitigation secondarily would help too. After a couple attempts you would be able to determine if you needed to put some more stamina on.
I'm not sure how you got 28% though Accident? If 3 levels higher is 8%, and same level is 5%, 9 levels would be 14%? (I got 16% because I was still working off 8.6% for +3mobs
) I know spellhit doesn't scale linearly, but as far as I recall, melee hit does, because it's simply based off the mobs defense skill.
One thing to keep in mind is that no CC would be reliable. Rogues would be spotted before they could sap, and spells would be pretty much guaranteed to miss. Everything would need to be tanked, which does mean more incoming damage. I suspect you'd need an MA and an MT due to that, as otherwise kills could go quite slowly due to the tank having to expend resources holding aggro on all mobs rather than just one. 2 healers might be enough. With an old school rotation and good 'not casting' mp/5 mana wouldn't be an issue at all, but throughput might. You'd have to strike a balance I think, with someone just refreshing hots and the other hotting and direct healing, then switch when mana on the direct healer gets to about 50%.
It's hard to say though honestly. It's possible you could just walk in there with 2 druids and 3 hunters, all stacking hit rating except 1 druid, and own the place. Who knows how hard a 26 elite hits a level 19 druid here? Or how long 3 level 19 hunters take to kill 3 level 26 mobs?