Can I rewire a 240 V 3 phase steamer to a 240 V 1-2 phase?
Yeah, it looks reasonable.  It was originally wired for 240V 3-phase "delta", with 2 separate banks of contactors hooking up heaters 123 or 456,  and all that fed through the 3-channel overheat protector. 

The new wiring is for USA style single phase: Both contactors (all 3 poles each) switch phase  L1 in parallel, connecting heaters 123 and/or 456, and current returns via the 3-channel overheat protector (paralleled) to phase L2.  

Obviously the machine was built with both these configurations in mind. 

With the new wiring, it would work on US style 240V split-phase.  It would also work in theory on EU style 230V single-phase.  

However as a practical thing that would probably not happen because EU power companies wouldn't want you drawing that much from one phase, They actually bring 400V 3-phase "wye" (230V to neutral) right to the pole in front of your house, and would bring 2-3 phases to a well equipped house.  This "wye" configuration would be yet a third diagram. 

But I doubt it's in Europe.  240V "delta" is silly when houses get 400V wye. 

I agree 9kw equals slightly under 40 amps, which in standard installation practice in North America would require a 50A circuit (6 AWG wire, good for 60A based on NEC 310.16 tables, I would still use a 50A breaker).   If you could meet all the criteria for 100% factoring, you could snake it onto a 40A/8 AWG circuit.