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So a druid would need:

Tower shield proficiency as a feat

An ironwood tower shield

Enchantment of Wild +1 (which is really +4)

Would I be able to use this shield before wildshaping, and have the shield bonus 'meld' into my new shape?

Yes, the druid gets the shield bonus to AC while using wild shape.

The Wild Shape class feature is a polymorph effect, which causes the creature's gear to meld into their new form. Normally, armor and shield bonuses don't apply.

When you cast a polymorph spell that changes you into a creature of the animal, dragon, elemental, magical beast, plant, or vermin type, all of your gear melds into your body. Items that provide constant bonuses and do not need to be activated continue to function while melded in this way (with the exception of armor and shield bonuses, which cease to function).

However, the wild special ability negates that last clause. Therefore the armor or shield continues to provide an AC bonus while the polymorph effect is active.

The wearer of a suit of armor or a shield with this ability preserves his armor bonus (and any enhancement bonus) while in a wild shape. Armor and shields with this ability usually appear to be covered in leaf patterns.

The +1 wild tower shield would give a +5 shield bonus to AC. Normally it also has -1 ACP and a -2 penalty to attack rolls. But wildshape with a wild item only preserves the bonuses, so you can ignore these penalties.

No, but you're probably supposed to be able to.

The rules are messy here. According to the description of the Wild magic armor/shield ability:

The wearer of a suit of armor or a shield with this ability preserves his armor bonus (and any enhancement bonus) while in a wild shape.

Taking the rules literally would indicate that the shield bonus isn't preserved, as it's not an armor or enhancement bonus. You would gain no benefit from the tower shield, and would essentially be wasting 16k gold and a feat.

That said, it seems likely that this is an oversight (though admittedly, Paizo copied the ability's text verbatim from 3.5 and didn't see fit to change the wording, so maybe not). It may be worth asking your DM if they'd be willing to houserule that Wild shields confer their AC bonus on a wild shaped bearer.

On a side note, if you're not planning to do any combat outside of wild shape, the proficiency feat isn't necessarily required. When transformed, you aren't technically wielding a tower shield, you're merely gaining the AC bonuses which would be granted by it. As a result, you suffer none of the drawbacks of wielding it, including nonproficiency penalties. However, if you're already relying on your GM's generosity in allowing the shield to apply in the first place, it's probably best to not push your luck.