#1 I recall the code base is different for the original series and the latter E/X series, i.e. you cannot use one's image on the other. I also recall 12.2(46) being a bit problematic. The most stable train in the 12.2 was (55). Yes, you should be able, if it fits on flash, to run any 12.2 version on a original series 3750.
#2 For the same exact version, functionally I believe there shouldn't be any difference.
#3 Depends on the switch and IOS version but typically things like tunnels, NAT and "weaker" QoS.
#4 Again for any original series 3750 (including the V2 models), I don't believe any have any license enforcement.