You've summed up the written rules well enough, so I can only tell you what I do.
I wouldn't rule holding two weapons gives you extra attacks, holding a weapon in you off-hand only allows you to use it without taking a round to switch to it (As you noted ambidextrous and multi-arm mutations give extra attacks. I base this on the fact the Knife Fighter Skill states the users get two attacks and if they have ambidextrous they get 4 attacks, 2 per hand.
That said I'd consider letting a player who intends on using an offhand weapon an "off-hand" version of the weapon skill, that if they raise to 3/4 they get it as an extra attack (off hand penalty still applies) but they must use it as off-hand weapon and it only applies to that one weapon type.
Weapons with a rate of more than one VS multiple targets, that depends on the weapon and the location of the enemies. Let's say you've got an auto-pistol (rate 2) and there are two enemies next to each other I'd allow it but if they were far apart I would impose an SV penalty (based on the called shots list TME-2-9) and maybe a reduction to the fire rate if they too far apart. As a loose guide, So if they have a weapon with a rate of 10 (chaingun) they can get the first hit at full SV, then they can move their aim to the next target 8m's away at 60% of their SV. +++ Originally I let them have as many attacks at full SV as their rate against every target in range but a character with a chaingun could clear most encounters alone 10xd20.