Playing a new game, got a druid/cleric for each of the spheres, all good.

Except, for some reason the Earth Druid has spells from the fire sphere.

Anyone else encounter this? Any solutions other than making a new character?

I'm confused. I thought Dark Sun druids aren't dedicated to a specific sphere like clerics are and receive spells from all spheres. (Of course, that may be the answer to your question. You selected your sphere (earth) for your character's cleric class, not their druid class. This means you will have access to more earth spells than spells from other spheres but will have access to some other spheres' spells that a pure earth cleric would not have. Also, keep in mind clerics and druids largely draw from the same priestly spell lists, so you have to be careful to disentangle which spells come from the cleric class and which from druid class. Note that if you are used to a later edition of D&D---Dark Sun uses 2nd edition---you might be further confused. Some of the rules changed significantly over time. Finally, remember that Dark Sun has a Cosmos sphere for all priests, cover 1st to 3rd level spells.)

In any case, as I recall the Dark Sun computer games (which I last played long ago), you will want access to a variety of spells simply to have a functional party. In particular, it seems to me that some cleric spheres are rather deficient in healing spells.

In table top:
* Clerics get minor access to Cosmos, and major access to one elemental sphere. (Note that Cosmos contains the spells you'd expect a priest to have that are not associated with an element; hence the decision to give them only minor access to Cosmos (which means their only healing spell is the weakest one, aside from a couple water spells that were printed at some point) feels rather puzzling to me.
* Druids get major access to Cosmos (making them better healers than Clerics once they reach 7th level), major access to an elemental sphere, and minor access to a second elemental sphere.
* Templars get major access to Cosmos and all 4 elemental spheres.
* Not sure about Rangers, but probably minor access to Cosmos and an elemental sphere. (But note that major version minor doesn't matter since Rangers don't get spells above 3rd level.)

In the computer games:
* Clerics are the same, but note that they lost the ability to gate in their element, making them feel rather useless.
* Druids lost their minor access to the second elemental sphere.
* Templars aren't available as a class for player characters; hence, I don't even know if templars are implemented as a class. (It's quite possible that templar enemies could just be coded like monsters, without having class tables, which would mean no XP tables.)
* Rangers actually get major access to the chosen elemental sphere. This means that a Ranger/Cleric dual-class actually gets major access to 2 elemental spheres. (Doesn't work with multi-class, because you can only choose one element at character creation, but I'm wondering how WotR handles dual-class humans that have been reincarnated into another race.)

This one's definitely a Druid, I chose earth for him and his "Druid" is in the brown colour for the Earth Sphere. It's just that his spells are clearly fire. Summon Fire Elemental, Wall of Fire, Flame Strike, etc. The same as my Fire Druid.

Also:
Clerics gain access to Turn Undead, Druids don't.
This is IMHO the main reason to even choose a cleric over a druid. Instantly deals 9999 damage to any undead. Very effective vs templars/defilers who like to surround you with a bunch of summoned ghosts, or skeleton warriors.

About your ranger/cleric reincarnation:
Given how janky Dark Sun 2: WotR already is, I would not expect anything less than a game freeze or crash. But please test it, for science!

Turn Undead, however, is highly situational. In battles where there's no undead (which, I'm guessing, is the majority of them), it's useless.

Thing is, to get Turn Undead, you're basically giving up *all* healing spells except for the 1st-level one (which is extremely weak) and those of water (which requires giving up the spells of any other sphere, unless you use that trick I've mentioned); furthermore, you'll have very few spells to choose from of levels 4 and higher.

As for Reincarnation, I haven't fully tested the human -> non-human case. But what I do know is this:
* Stats like HP don't chance (in case you chance from/to Half-Giant).
* I don't remember if thri-kreen gain/lose their extra attacks.
* I'm not sure what happens if your level exceeds your racial level cap.
* In my testing, I never had a non-human become human, which is rather curious.

By the way, I would expect most spells that you can actually get to be at least somewhat functional and not crash the game; this doesn't hold true for 8th and 9th level spells. Also, one WotR bug (not in the first game) is that, if a character's HP is already above its maximum, healing will increase HP further; with the help of a Half Giant with Lend Health (in WotR, it's a Psychometabolism wild talent) and someone with Heal/Water of Life, this could be used to get someone very high HP.

As for the 8th and 9th level spells, which you can get with the cheat that gives you all the spells, they vary in how well they're programmed:
* Meteor Swarm allows you to target 4 fireball area attacks. These do not need to be near each other. This spell is probably better than in table top.
* Time Stop gives you infinite turns. The catch, however, is that your movement doesn't recover between turns. If you are unable to reach the remaining enemies, it's a softlock because there's no way to end your turn so that someone else can act.
* Prismatic Sphere outright crashes the game.

Edit: Also, with healing, why are there no Cure Wounds spells for 2nd *and* 3rd level? There's 2 consecutive levels without healing spells, and the healing spell you do get isn't adequate. It's something I've always wondered, and it's always felt like a design flaw to me (including in Wizardry 1-5, which copied the AD&D healing spells). It's the sort of reason I don't want to play Pool of Radiance, as you don't even get the second healing spell, resting doesn't bring you back to full, and there's no shortcut to bring your party back to full quickly (in real time), even if you're at a perfectly safe location.

"Aid": works fine as a 2nd level healing spell.
It's present in EotB and Baldur's Gate, as well.

Problems:
* It doesn't heal any more HP than Cure Light Wounds which, as I've mentioned, just doesn't heal enough.
* In tabletop, it can't mechanically replace out-of-combat healing, as the temporary HP don't stack, and will go away after a while. (Note that not all CRPG implementations are accurate in this regard; the Dark Sun CRPG implementation, for example, is not. Dungeon Hack, on the other hand, implements this accurately, and I think Temple of Elemental Evil might.)
* It's not in Pool of Radiance (the particular game I mentioned that has a serious issue with not enough healing), and the Wizardry series doesn't have a corresponding spell (unless you count Wizardry 8's Guardian Angel, but that game's Heal Wounds spell is strong enough and can be cast with more power).

By the way, in the Dark Sun CRPGs, healing spells can be cast at infinite range, which gives them more in-combat utility (like if you can't reach any target you might want to attack).