My memory is not what it once was.
But as i recall, the 7400 was the first G4. The 7410 was the second G4, or as moto called it, the G4+. The main difference was the L2 cache, and i believe an increase in stages of the pipelines, maybe from 4 to 7, i cant remember exactly. Also a shrink in die size and it was fabbed at 0.18 microns.
The 7410 was really a stopgap from moto, after the ultra-embaressing 7400 being stuck at 500Mhz for a year, they needed something quickly. The 7410 was it, but at the time there was controversy as to whether it was actually much faster than the 7400 in "real-life", in some benchmarks it lost out i believe. But then, in some benchmarks the Pentium3 actually loses out to a 166 (i dont know whether thats as unbelievable as it first sounds).
Whilst the 7400 and 750 are very similar, generally the G4 is actually derived from the 604 rather than the 603. Though again, this isnt entirely accurate.
At the time altivec was a big thing for apple, and as apple saw it as a next generation of cpu, i guess they decided to call it a G4 rather than a G3. It still is a remarkable piece of engineering. Imagine at the time of Pentium3s and Pentium2s and MMX instruction, to hear that a G4 had a VPU unit that could process (in theory) a set of 16 instructions in one go. As opposed to a pentium, which did it in 16.
Yes the G4 has full MERSI support. There cannot be a dual G3, unless IBM come up with this full MERSI compliant 750 that the rumours have suggested.