Organics of Transmetals & Transmetal 2s?

[Ode to a Grasshopper]

And as for melding Sparks - I see it more as BW Optimus and BW Megs simply taking a 2nd Spark into their body, rather than the Sparks merging. In Optimal Situation, we see G1 Prime's Spark sitting in the cockpit section of Primal's form, where we later see Primal's Spark housed within his body. I'm of the opinion that Sparks would remain separate of each other.

I would have thought so too, but Megs is pretty clear about 'mingling' Sparks and Primal takes OPs spark into his own Spark chamber...until his body finishes upgrading anyway.
It's quite possible (albeit just more fanwank/conjecture) that the sparks of OP and Megs (being both 'guzzler' sparks and exceptionally powerful ones at that) mingle initially until their host bodies are capable of holding them and then return to being separate once it's 'safe' to do so.

Are Transmetals actually Organic? There's no doubt the robot modes of TM forms have organic motifs, with Cheetor and Primal appearing slightly furry. Rattrap however has no such detailing - all he has is the appearance of muscles or musculature - which is also seen on Optimal Optimus beast chest. So the argument could be made that yes Optimal Optimus is still retaining some organic nature.

Again, my first thoughts are the same as yours, but the TMs do apparently have a little bit of organic matter in robot mode - even if it is mostly chest musculature.

There's one very important aspect that you almost touched on, but didn't quite get to:

BW Megatron becomes a TM2, instead of remaining a TM.

Every other TM2 (Cheetor, Dinobot and Blackarachnia) all have some contact with the Transmetal Driver in attaining a new form.

But Megs doesn't.

You could speculate that perhaps BW Megatron did some initial testing of the Driver on himself, before attempting to create Dinobot II, and thus had residual energy within him when he took on G1 Megatron's Spark.

Nice...that provides a decent explanation. When the not-quite cloning device explodes in Feral Scream part 1 Megs is present but Optimus isn't. After the Transwarp wave energy having an effect on existing- and protoform-Transformers (at least Predacons and Maximals) that works re. 'infecting' Megs, albeit on a cloning-machine VS Planet-Buster scale.
I salute thee sirrah.