A 97:

If you're only using sudden metamagics with a spell, then there's no interaction with Arcane Thesis, because Arcane Thesis can't decrease the spell slot adjustment of a spell below zero. If you're a sorcerer combining it with conventional metamagics, then it provides the same benefit as a +0 metamagic like Invisible Spell: Each sudden metamagic you apply will decrease the net slot of the spell by 1, to a minimum of its normal level. If you're a wizard, there's again no benefit, because sudden metamagic is applied at the time of casting, too late to do anything with conventional metamagic applied at time of preparation.

A 98:

There are at least three. PHB2 has an alternate class feature for sorcerers called Metamagic Specialist (replaces familiar), which lets you apply metamagic without a casting time increase. Complete Mage has a feat called Rapid Metamagic, which does the same thing. And Complete Arcane has the feat Arcane Preparation, which allows a spontaneous caster to instead prepare spells in some of their slots, allowing metamagic to be applied at time of preparation, like a wizard.

A 99:

Sorcerer would be a non-associated class for frost giants, since it doesn't particularly play to their strengths (melee combat, no Cha bonus), and 7 levels is less than the giant's 14 HD. Thus, each level increases its CR by 1/2, so a frost giant sorcerer 7's CR would be equal to 9 (as normal for a frost giant) plus 3.5 (for 7 non-associated levels), or 12.5 . I don't remember if there are any rules for rounding fractional CRs; if there are, this would round to 12.