It doesn't need higher revs to charge. First of all it doesn't always charge when you hit charge button. It charges whenever it can at a given point in time at given driving conditions (throttle position, drive rpm, drive load, power requirement.....) Generally when all power produced by ICE is needed and/or used by PHEV it doesn't charge but as soon as conditions change (such as load/power req. goes down, change in throttle position) it starts charging. This is also the answer to your question in the second paragraph.
That's interesting and, having never used Charge myself, goes some way towards answering my question. However, I still can't understand, if there are times when the ICE has spare capacity to run Charge mode, why would Mitsubishi have not made this the default option, unless of course it does use more petrol? Perhaps it does use more but the penalty is just not as great as most of us had assumed (which is great news if true).
Look forward to hearing the results of your tests Maby
Hats off to Mitsubishi, the more that you think about it, there's lots of clever engineering gone into this car. Just a shame the MMCS is such a weak link.