Hi.
Here are my 2 cents on your questions:
1: Yes, spells can be converted into touch attacks. This is more of a house rule. The effect of the spell will be the same, only changing the "ranged" or "sight" factor into "melee touch"
2: The effect of the spell would be the same, unless you are trying to combine it with a weapon, for example, a stun knife. The weapon would then become a Foci, and you just do melee attacks. Now for the resistances, it would be two different rolls, one for the spell effect, and one for the knife damage value in this example.
Touch effects also bring into consideration "glance attacks", which is also another house rule call. Normally if the defender wins or ties the "dodge roll" he avoids the attack. On touch attacks, a "tie" means a "glance hit", on which case the effect of touch weapons, like stun batons or shock gloves, still take effect. (page 149, action turn 3 of SR4 book)
3: As for drawbacks, other than the standard maintain penalty, the big one is that the mage will need to be in "melee" range. Which as all you know, is a place magi reeealy don't want to be in.
