Can I use HFS-2 instead of HFS-3 for my 335i setup ?

If you don't have a way of manipulating the timing and fueling via a Piggyback, then you can dump boost to wastegate pressure with the HFS-3.

These cars do tune themselves to an AFR with what I call the lambda map, so if the injection were to stop, the car would correct itself to some degree. The issue is how fast would this happen and would any damage occur in the transition. This and your right foot would be the failsafes with the HFS-2.