It's ultimately a thermal limit. Torque will also be reduced.
This whole scheme rests on the observation that used oversize 3-phase motors are cheap, so one can afford the necessary overkill.
So, run it at 1/2 to 2/3 of rated load, and be happy. Note that this is the actual load, not the max the metalworking machine will allow - HSMs rarely load their equipment to the levels of production machines.
Not directly. The point is to understand how motors work.
The best source for information on driving 3-phase motors from single-phase supplies is the patent literature.
Joe Gwinn