I hope the survivor makes a good recovery.
Three points that might or might not be relevant:
1. With a constant speed prop, the engine can often make rated rpm but produce little power. Manifold pressure won’t tell you. You have to check fuel flow to confirm you are making rated power. Flow would be about 23l/h at 5800 full power. Mere engine noise won’t tell you much. Furthermore, if the prop is installed and adjusted correctly, you should be able to take off on the full coarse pitch stop at the cost of overloading the engine.
2. When the aircraft takes off it is flying relatively easily in ground effect until it reaches about 1.5 times wingspan. Climbing above that depends on power available and aircraft weight.
3. I have a 912iS, not yet flying, it has lots of redundancy. About the only things, I think, that might go wrong absent complete mechanical collapse, are air and fuel flow. On my aircraft, shutting the fuel cutoff stops the engine in about two seconds. Rotax normally supplies two fuel pumps in one assembly. Provided both were on for takeoff there shouldn’t be a fuel problem unless there was a restriction like a clogged filter(gascolator and high pressure filter) or water somewhere. The pumps and ignition use generator A and if that fails there is instantaneous switch over to generator B so if the engine was turning the pumps were running.
Assuming the Aircraft had an EFIS, you would expect voltage and fuel pressure warnings.