Myrrh, Frankincense, and Steel: Kurald Galain's Guide to the Magus

[Kurald Galain]

Can you use TWF or MWF with Ranged Spell Combat?

As Casualviking states, this doesn't work. I suggest looking into the Natural Spell Combat arcana to get more attacks with spell combat.

Since I'd also like to be able to cast offensive spells (and not only snowball) in combat against high CR monsters

Empowered Intensified Snowball is pretty good against high CR, particularly if you dip into sorcerer (as it bypasses saves, SR, and DR). Not many monsters are cold-resistant. The Magus's enchant weapon ability makes him very good at piercing DR in general.

What are the best ways for a magus to boost spell save DC?

Same as a wizard: Spell Focus and int boosts, basically. The Magus is generally better off maxing his strength (or dex) and not using spells that require a save. At level 15, you're already three points behind a full caster (because he's casting higher level spells than you) and the caster can afford a higher primary stat due to SADness.

Are any of the Rare Cantrips worth picking up? Jolt seems like it'd be particularly useful for a Magus, giving all-day Spell Strikes.

There are several other cantrips that already give him all-day spell strikes, such as Arcane Mark.