2. Basically what I'm concerned with is having set up at eg. 50RPM lean of peak, then playing around with the throttle at the same altitude (& ambient conditions), will it be always 50RPM lean of peak at every other throttle setting?
Yes; that is basically correct.
If you set the mixture to around say peak EGT, then you can vary the MP quite a lot and the engine will remain around peak EGT.
The exception is when there is a big altitude change e.g. in a descent from FL150 to 3000ft you will need to enrich the mixture, because if you were at peak EGT at FL150, and then descend, the engine will go leaner and leaner and long before you are at 3000ft it will be way LOP and will probably start running rough.
I have a writeup on non-turbo fuel injected engine management
here. It is quite long but you will get the general idea.
I don't bother with LOP; it does not produce any more MPG, except as a side effect of delivering less power and flying slower as a result of that
And even with GAMI injectors my engine does not sound that smooth when LOP.
Regarding the original Q, it is correct that one should not lean a Lyco motor when the power setting is above 75%. However, this is a crude guideline because, following takeoff, the power drops
continuously as one climbs, and it is obvious that there is nothing inside the engine which will suddenly wake up at 75% and blow it up
The constant-EGT climb technique is generally accepted as totally safe, and it delivers an easy method for climbing all the way to one's operating ceiling without touching anything but the mixture lever.
What is really important is setting a reasonably high IAS
ASAP after takeoff, to avoid exceeding ~400F on any cylinder. For me (TB20) that means trimming for 120kt pretty soon. And sometimes it cannot be done; for example I departed from some place in Turkey at +35C and could not keep it below 430F, but that is still OK. Some SID profiles also make it hard - look up some of the ones in the Alps, for amusement
If a SID has a 10% climb profile then it is probably a no-no for piston ops.