Can a Magus use Whirlwind attack to deliver a spell to multiple opponents with Spell strike?
And secondly, Is Whirlwind Attack even worth taking if it can't do the above?
From the Magus rules on PFSRD (emphasis mine):
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. ...
Spellstrike is a free action that is considered part of the action used to cast the spell.
From the Whirlwind Attack feat on PFSRD (emphasis mine):
When you use the full-attack action, you can give up your regular attacks and instead make one melee attack at your highest base attack bonus against each opponent within reach. You must make a separate attack roll against each opponent.
When you use the Whirlwind Attack feat, you also forfeit any bonus or extra attacks granted by other feats, spells, or abilities.
Whirlwind Attack is a full-attack action. You can't replace the free attack from Spellstrike with a full-attack. Whirlwind Attack also explicitly says that you give up any other attacks you might have from other sources.
However, some touch attack spells grant multiple charges (for example, Chill Touch). While the free attack you get when you cast the spell initially cannot be turned into a whirlwind attack, you can use any remaining touch charges during a later whirlwind attack.
Note that this still may not be a worthwhile combination, simply because Whirlwind Attack has a lot of prerequisites that are not particularly useful, and only pays off when you're adjacent to several foes (which is not normally a safe place to be).
Yes spell strike can be used with whirlwind attack, but spell combat cannot.
Spell strike allows you to deliver touch spells via your weapon. If you are holding a charge from a touch spell it would apply for as many touches remaining.
Yes. For example, if you cast chill touch (which allows multiple touch attacks), you could use spellstrike to cast and deliver the spell through your weapon, and in later weapon attacks you could use your weapon to deliver the remaining spell touch attacks (one spell touch attack per weapon attack).
If you have multiple attacks per round with that weapon (such as from having a BAB of +6 or higher), you can use the weapon to deliver multiple spell touch attacks per round, so long as you have uses of that spell touch attack remaining.
For example, if you are an 8th-level magus (BAB +6/+1) and you cast chill touch, you have up to 8 uses of that spell touch attack. If you make two weapon attacks in a round, you can deliver two spell touch attacks per round (one for each successful weapon attack).
Spell combat says http://www.d20pfsrd.com/classes/base-classes/magus/#TOC-Spell-Combat-Ex-
As a full-round action, he can make all of his attacks with his melee weapon at a –2 penalty and can also cast any spell from the magus spell list with a casting time of 1 standard action (any attack roll made as part of this spell also takes this penalty).
Since spell combat is a full round action and not a full attack action you are not able to cast the spell in the same round that you would use whirlwind attack. The exception being if the spell is a swift action or faster.