[RPG] Can Magus change their grip on their sword during spell combat as a free action

You probably need to read the action type and concentration rules. Most of this isn't "magus rules" it's "general Pathfinder combat rules."

You get one full action, or one move and one standard action, a round, plus a free action. You can 5' step at any point before/after/during any action if you didn't move otherwise with your move action. One attack is a standard action, multiple and many special routines are full round actions.

1. The magus' spell combat ability means he can cast a spell at the same time he attacks as a full round action.

2. The magus' spellstrike ability means he can cast a touch spell as a standard action and then, as usual for a touch spell, gets a free action to attack with the spell, but as a special bonus can use a melee weapon attack to deliver it.

Your examples above are unclear as to if/when you think other attacks may take place.

You may cast a touch spell (standard), move (move), and attack with spellstrike (free). You may not cast a touch spell (standard), move (move), and perform other attacks (standard/full). You also may not use spell combat (full), move (move), and perform other attacks (standard/full).

Your first two questions say melee attacks plural, so

1. Yes, you can cast, 5 foot step, and make all of his attacks at -2 plus the free spellstrike attack. Or cast and make some attacks/5' move/make other attacks. All legit combos as part of a full round action.

2. No, you may not cast, move, and then make any attacks other than the free spellstrike attack. Note in this situation you can't cast and attack/move/make free attack either because spell combat is a full round action. You have to use spellstrike only in this scenario.

The only difference standing next to someone is the threatened aspect. You can choose to not cast defensively and take an attack of opportunity; if hit you have to make Concentration DC 10+dmg+spell level or lose the spell. Or you can proactively cast defensively, make a concentration DC 15 + 2xlevel, lose the spell if you fail.

All your last scenarios just need to understand that the AoO or Concentration check doesn't lose any of your normal attacks, but if failed loses the touch spell and the free attack for the touch spell. It seems like you're trying to cheese getting the free attack even if your spell is disrupted, that is not the case. Oh, it also seems like you're unclear on spellstrike - the spell is conveyed by the melee attack. You hit and do melee + spell or you miss with both.

1. Make/fail melee attack, fail at concentration check, make free melee attack? No, you only get the free attack as a result of casting the touch spell.
2. Make/fail melee attack, succeed at concentration and do spell damage, make free melee attack and do damage? What? You deliver the spell damage as part of the free attack, if you hit.
3. Make/fail melee attack, succeed at concentration check, make free melee attack and do melee + spell damage? Exactly.
4. Make/fail melee attack, succeed at concentration check, Fail free melee attack and do no spell and no melee damage? Correct as well.
5. Make/fail melee attack, succeed at concentration check and do spell damage, make free melee attack and fail? No, the spell damage is delivered by the attack.
6. Make/fail melee attack, succeed at concentration check, Fail free melee attack, make free touch attack roll to deliver concentration spell? No, the spell damage is delivered by the attack.

Per this Paizo FAQ post, your spell strike attacks are like any other touch spell - missing the attack still holds the charge, and you get to try again till discharged. But none of those are "bonus" actions. It means "you know, like next round, when you get another attack." Just like other touch attacks - you miss with your free attack, you can still deliver with other standard actions later.

So, for example, you can, in one round: declare spell combat and spellstrike; cast a touch spell as part of your spell combat full attack, then five-foot step into a threatened square, make your free attack (from spellstrike) to channel the spell and fail your attack roll, but then make your other melee attack (from spell combat) and if it succeeds, it will channel the still-held touch spell.

Your GM is correct. Spellstrike doesn't give you a free cast, what it allows you to do is turn your melee touch attack from casting a spell into a normal attack, allowing you to deal your weapon damage in addition to the spell.

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.

Spellstrike Description on d20pfsrd.

So assuming I've got all my rules straight (someone please double check me on this) you would get a total of 5 melee attacks (4, 4, -1, 4, 4) plus one spell touch affect if at least one of those melee attacks hit. You use spell combat to cast a melee touch attack spell, which spellstrike turns into melee attack at highest BAB. You get your normal attacks, and your attacks from Speed and Sudden Attack. Since missing doesn't cost you your touch attack charge, you can keep trying to deliver your spell with each attack.

As Louis Huppenbauer pointed out, both using the Arcane Pool and Sudden Attack are swift actions, so making 5 melee attacks relies upon you having applied Speed in a previous round (it has a duration of 1 minute).