For Anyspell, do you believe that a Druid/Contemplative taking the Spell domain would be able to prepare an arcane spell in the (single) 3rd level slot where they can prepare a domain spell? I'd say "yes" and I expect that most would agree.
I'm not familiar with Druid/Contemplative. Are his spells listed like: 4+1, 3+1, 3+1, 2+1? If so, then yes. If there is no "+1", then the answer is No. You can cast the spell from any source, but you need to have "+1" in your 3rd level available spells.
BTW, it IS possible to have access to a domain but not be able to cast any of the spells. Access to and ability to use are two different things. Remember, most domains come with a perk just for knowing it.
For Arcane Fusion, I just double checked the rules for Archivist. There is no rule stating that a spell scribed by the archivist ceases to be a spell on the source list. Hence it remains a valid target for Arcane Fusion by default.
I don't care if it's on the spell list. You are confusing spell lists with class spells.
Example:
I'm a sorcerer. I forget the spell but my cleric friend does that thing where you loan a spell. Now I have cure light wounds "memorized". Can I cast it in Arcane Fusion? Nope. I don't know it, so it's not a sorcerer spell.
I go with the BS to become a sacred Sorcerer. I now have access to the sacred list of spells. Now, by YOUR logic, I cannot use Arcane Fusion with any of those spells. However, those spells are added to MY spell list. Not the spell list of every Sorcerer on the planet. They become Sorcerer spells, For ME, and me alone.
See, the problem is how they worded it. They said it allows you to select Sorcerer Spells. They didn't say, "From the sorcerer list." Well, what's a sorcerer spell? A sorcerer spell is a spell cast by a sorcerer. If I figure out how to add cure light wounds to my list of spells know as a sorcerer, when I cast it, is it clerical? Is it Emissary of Barachiel? Kitsu School? Paladin of Tyranny?
No. They may have access to the spell, but this particular spell that I am casting is Sorcerer, because I'm a sorcerer and casting it.
I have to be a sorcerer to CAST a sorcerer spell. I can be a wizard, and cast spells that are from the sorcerer spell list, but the spell itself is a wizard spell, because that's the class I am using to cast it. A wizard using Arcane Fusion cannot select a 1st and 4th level sorcerer spell any better then an Arcanist.
Now, if they said, "From the sorcerer LIST" That would a whole other can of worms.
A sorcerer can't generally use a prayerbook both because the spells are typically not sorcerer spells and because the spells are always divine. But if an archivist scribed a divine sorcerer spell into their prayerbook and then created an arcane version of the same spell on a scroll? The scroll could be used by a sorcerer as it has an arcane type and is on the sorcerer's spell list.
Well, yes. But not for the reasons you think they are. An archivist using Alternate Source could scribe an Arcane version of a spell from the Sorcerer List onto a scroll. He could read it, because it's on his spell list. However, the spell itself would be an Arcane Archivist Spell, because an Archivist scribes it.
Similarly, a Druid spell scribed into a prayerbook, memorized, and scribed onto a scroll could be used by a Druid according to scroll use rules: http://www.d20srd.org/srd/magicItems/scrolls.htm.
A spell list says what spells you can use off a scroll. Using a spell doesn't change who scribed it.
It doesn't come up very often, but this is one of those times. Here...
Magic missile
Force Domain 2, Knight of the Weave 1, Nentyar Hunter 1, Sorcerer/Wizard 1, Warmage 1, Wu Jen 1
Now, it's a arcane spell and divine, depending on who's casting it. But, Magic Missile is NOT on the cleric spell list. If I had full access to the cleric spell list, I could not cast this spell, even if it was scribed on a divine scroll. I need access to the Force Domain spell list before I could cast it directly from the scroll without a UMD roll. Sure, it's a divine spell, but it's not on my spell list, I cannot use it.
Now, as an archivist, if you find someone with the force domain to scribe you a scroll, you can then copy that spell over into your spellbook. When you cast it, it is now an Archivist spell. As you cast it, it's Archivist, NOT Force Domain. Get it?
Sorry, but using Arcane Fusion as an archivist isn't RAW.