Uh, with Spellstrike, you can just deliver all those Chill touches through your weapon. You don't need to use Spell Combat at all for that.
Can you explain why? It doesn't change the damage or anything--actually it deals more damage on average with the weapon because of the extended crit range (you did take a weapon with extended crit range like a scimitar, right?).
With your method, you're taking a -2 penalty to both attacks in order to potentially deal the same damage.
I see no benefit to what you're proposing.
Ok, but why should there not be the option to spell combat.
Note: Spellstrike is not directly involved. My suggestion would also allow spellcombat without spellstrike using a spell charge touch attack as off-hand weapon.
A Magus gets spellstrike at level 2. Your change would matter for 1 level, when you wouldn't get multiple touches anyway. Why bother?
e+f) do change the potential dmg compared to your suggestion being b). An additional attack is gained for -2hit just as with TWF.
I think this is the issue--you're misunderstanding the Spellstrike.
You hit with the weapon and discharge the spell when you use Spellstrike.
For example, lets say you're a level 2 character using a Scimitar with Dervish Dance and a +4 Dex modifier (that's pretty typical, but you could make it a Scimitar with +4 Strength mod just as easily).
On a normal swing, you deal 1d6+4 damage. A Shocking Grasp deals 2d6 damage.
So, going through your scenarios, you'd deal the following damages:
a) 2d6
b) 3d6+4
c) 3d6+4 (though at a -2 to hit) and that other spell
d) 6d6+8 (though you need to hit twice, each at a -2 to hit)
e+f) 3d6+4 (though you need to hit twice, each at a -2)
The spell adds to your normal attack with Spellstrike, it's not spell or strike.
e) off-hand: d6+4 +2d6 + main: d6+4 = 4d6+8 (need to hit twice at -2 both vs armor - not using up another spell)
Where is 1d6+4 damage coming from on your off-hand? You explicitly need a free hand to use Spell Combat.
Do you have a monk level or something? Because if that's the case, what you're talking about is perfectly legal--though you'd need the Two Weapon fighting feat.
Wait, so you want to give the Magus a free touch when they cast the spell and when they have a held charge?
I think that is way too strong for the first few levels, and completely pointless and weak by the teens.
I think Magi are of a fine power level. Do you perceive some imbalance or weakness you're trying to compensate for, or is this just a, "Wouldn't this be awesome if my Magus kicked even more ass?!" thing?
I think it comes down to whether you see the Spellstrike as a) a melee attack with a spell augmenting it, or b) a spell that is delivered through a weapon.
As I see it, when you cast the spell, the Spellstrike functions as a spell that is delivered through a weapon (b), allowing you to use your weapon's bonuses and crit range and add your weapon's damage to the spell's effect.
However, when you're holding the charge and Spellstriking, you're effectively just making a melee attack that is augmented by the charge of the spell (a).
While this seems like pointless garbage (and it is for almost all intents and purposes), it becomes very important when you want to combo a held charge in spell combat.
Spell combat as written allows you to cast a spell and attack with a weapon in the same round, as though using a "2 weapon fighting" variant. Now, if you think of the Spellstrike attack with a held charge as a melee attack (a) (which it effectively is, by RAW), then that would be the melee attack part of the spell combat, so you would have to cast a spell with the other hand. (ie. you don't get 2 melee attacks without casting a spell in between)
If you hit with the Spellstrike charge, then you could follow it up with a spell in the other hand and use Spellstrike to channel the spell through your weapon.
As I see RAW now, you couldn't use a held charge as the "spell" part of the spell combat, however I don't think that it would make much of a difference at the end of the day (getting a house ruling should be easy). As you said, it would be pointless at higher levels, and useful at lower levels for the extra attack. That said, I think the magus is pretty weak at the levels where an extra weapon attack would make enough of a difference to consider it, so I think it should be fine.
For my 2 cents I would argue that it works. After all this isn't even a particularly effective use of the class. My magus is level 3 now and chill touch does not look sexy at all.
Until you reach a BAB of +6 or get hasted - if you cast chill touch with Spell Combat and Spellstrike you get 2 attacks (and chances to deliver a charge) in the first round and a whooping 1 attack in every round after that - since casting a new spell with Spell Combat and Spellstrike would end your chill touch.
My Magus does, and will use spell combat and spellstrike ever single round in combat if possible. the extra attack and the spell damage (even if brand only does 1 HP damage at the best of times).