Having you considered getting rid of advanced learning (or letting the healer choose between a domain and a doubled advanced learning at 5th level and every 5 levels afterwords)? Although it seems good to do it and keep in step with other healing specialists, your domains pretty much do the exact same job and then some.
Restricting the metamagic effects to healing spells is probably a good idea (if I'm reading your first suggestion properly).
Combining your second and third idea, perhaps keep spontaneous domain access but replace the first level domain with spontaneous healer? Giving the cure domain wouldn't grant any new spells known and though the boost to the healing dice is nice, healing hands seems like more than enough for 1st level.
The last idea sounds interesting. I'd definitely say yes to death/undeath but I could imagine a healer of Hextor/Heironous wanting the strength/war domains to serve on the battlefield so... do as you see fit.
Also, have you considered giving this class a way to sneak in buffs to allies along with heals (beyond temporary hit points) such as through temporary buffs like field medic from heroes of battle or through a reversed duskblade channeling ability (letting you target someone targeting by your healing spell with an additional spell).