As far as the question regarding military training on piston helis goes - we don't train on pistons because the contract was let several years ago when the Defence Helicopter Flying School (DHFS) was set up, following concerns that 3 Services, Army Navy and RAF, were training on the same aircraft (Gazelle) at 3 different locations and thge asumption that therefore there should be some economies of scale to be gained by combining them.
Then the decision had to be made on which helicopter type and which contractor to provide the aircraft and engineering support.
Now you will ask why the piston helis didn't get a look in, and it's mainly because no-one wanted them. At the time the only option was the R22 which had such a horrendous record in terms of training accidents (dynamic rollover, mast bumping, running out of power) that nobody, who knew what was involved in Military flying training, would have looked twice at them.
Just for once, in Military procurement, the bean counters didn't get their way; it would have been very easy to make a financial case for using R22s instead of Squirrels purely based on operating costs with no regard given to performance. One of the requirements of the desired aircraft was that it should be capable of flying at 120 kts which clearly ruled out the R22. Actually it should have said cruise at 120 and dash at 140 but someone missed that bit out and the DHFS Squirrels are torque limited to preserve airframe and engine life to the benefit of the contractor.
I don't know what the R44s top speed is but even if it was fast enough it is still a Robinson and as such is better suited to being gently bimbled round the sky rather than the hard life that the Squirrel (very successfully) endures as the workhorse of DHFS.
We don't spend that long teaching pilots to physically fly a helicopter, that is the easy bit and anyone who can drive a car can learn to fly a helicopter - we spend a lot of time teaching pilots to operate and fight a helicopter by day and night, and in order to do that we need an aircraft of increased complexity, ruggedness and performance.