a) already answered. Early mages are already extremely weak and being reliant on mana restoration aka investing points into mana restoration would mean that their damage would be stunted to the extent that it would probably be worse then even an archers.
b) Well the thing is your mana usage does scale by a percent being 200% every time you level up 100 levels. If a pre 100 character chose to invest enough points so that he/she is getting 600mana every 30 seconds I would imagine that he/she would be extremelyyyyyyyyyyyyy weakkk. The extended y's and k's are here to emphasize my point
c) Didn't talk about dex at all. Dex should be upgraded at around a level where investing points in magic gives less damage output then investing points in dex and crit damage thing overall. So yea that's kinda unrelated to the argument because I did not talk about dex at all. I was talking about much more necessary skills such as magic projectiles and then fixed mana and then range. The last 2 being somewhat interchangeable. Running to shops is an entirely negligible point that has no reason to be mentioned.
No idea what you are on about with the test dummy but like I said the percentage restored, like anything newly introduced to the game, must have some consideration to it.