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They can cast their touch attack before going into combat and hold the charge (or cast in combat, if needed.) They can then deliver both the effects of a regular unarmed attack, along with the spell's effects.
They don't NEED IUS it for touch attacks (as a spell charge counts as a weapon), but a cleric headbutting a zombie to a disintegrate-y death just looks cool.
You generally need the domain strike feat to combine an unarmed strike with a domain touch power, but RAW you can't do it with a spell as a cleric unless you multiclass as a magus I believe. That said domain strike with certain domains is very good.
Also IUS opens up a number of interesting feats like stunning fist, touch of transquility, and certain style feats (snake style is great for a cleric) and allows you to retain your melee attack capability while juggling spellcasting, metamagic rods, scrolls, and shields much more easily.
If you're holding a charge the next creature you touch gets the spell. However, if you're in combat and they're not going to let them touch you, you can either go with a touch attack for the spell (considered armed) or with a melee strike with your hand (not considered armed unless you take IUS) in which case you have to beat their full AC.
This is correct.