When holding a charge, whether because the initial touch attack missed or was not used, does Spellstrike allow for the use of the Magus' weapon for subsequent attempts? For instance,
Would the Magus then be forced to deliver the spell via a standard touch attack?
No, he can try again with his weapon melee attack. Per spellstrike,
At 2nd level, whenever a magus casts a spell with a range of “touch” from the magus spell list, he can deliver the spell through any weapon he is wielding as part of a melee attack. Instead of the free melee touch attack normally allowed to deliver the spell, a magus can make one free melee attack with his weapon (at his highest base attack bonus) as part of casting this spell. If successful, this melee attack deals its normal damage as well as the effects of the spell. If the magus makes this attack in concert with spell combat, this melee attack takes all the penalties accrued by spell combat melee attacks. This attack uses the weapon’s critical range (20, 19–20, or 18–20 and modified by the keen weapon property or similar effects), but the spell effect only deals ×2 damage on a successful critical hit, while the weapon damage uses its own critical modifier.
There are two separate clauses here - "he can deliver the spell through any weapon he is wielding as part of a melee attack" and "Instead of the free melee touch attack normally allowed to deliver the spell, a magus can make one free melee attack with his weapon." It seems to me that if the attack misses, the charge is held (just like if you were to miss with a touch attack on the same spell, it would be held) and you can try again with your own attacks, just like with normal spell touch attacks. The statement about the freebie doesn't say it's your only chance, in fact it takes pains to make it parallel with the usual free touch attack.
According to the rules as written in the Spellstrike description ...
Instead of the free melee touch attack normally allowed to deliver the spell, a magus can make one free melee attack with his weapon (at his highest base attack bonus) as part of casting this spell.
... I would say you can't try another Spellstrike attack with the same held charge. The description implies that you only have one chance to use your weapon to deliver the spell. If that attack fails, you're still holding the charge but can only deliver it with a standard touch attack.
You can deliver the spell in later rounds by using spell strike with your normal melee attack you make with your weapon. Example, if you have only 1 attack per round and do spell combat and cast shocing grasp you get to make a spell strike attack to deliver the spell if that missed your then make your normal attack and can deliver it via spell strike. If you missed during that round the next round you can use that saved charge and attack normaly and by using spell strike deliver it if you hit.
