I have to agree that Henri will be a last resort. If Ottokar brings him in to help deal with the Ravens, I imagine more than a few electors are going to wonder what the point of having an emperor at all is after the last one crushed them with casualties and taxes for foreign adventures and the current one failed to protect them from both external and internal threats.I'm not sure that Ottokar would be enthusiastic to bring in Henri though, because he knows he'll have to cough up his pride, his reputation, and the lands of the Rhine to him just to crush the Ravens, who are basically just peasant rebels. That's seriously not a good look for an Emperor who is pretty shaky in terms of his authority, so he'd have to weigh his options before clicking that "SOS" button to the Triunes.
Meanwhile, the Ravens are obviously a grave threat to the current social order, but they remain a much more subdued enemy compared to the massive raving armies of the Triunes and with them taking Magdeburg as a base, that only makes them an easy static target for the Germans in case they do need to get rid of them.
It'd be more interesting if Ottokar manages to crush the Raven's Rebellion on his own terms instead of taking the easy way out with Henri. The Ravens can be a useful tool for him both militarily and politically, being a weapon he can manipulate to crush the Triunes while giving some small concessions to them, only for him to betray them when they do not see Ottokar and Elisabeth's treachery. I can see Ottokar firmly establish himself as the HRE Emperor among the Electors and the rest of the German court through the death of the Ravens if done in this fashion.