Hmph, I purposely refrained from calling the whole soul plane thing an "all-spark" but someone's gotta go ruin it and drag Transformers in here huh.
To me it can be both the things you suggest though. Individual entities just get blurry on a soul level in my opinion. We might once existed as individuals there in the beginning if you believe in beginnings but we're connected, bonded to so many others that we start mingling with and the whole "who is who?" thing gets hard to answer. So all sparks do go back into that same mess of souls and multiple sparks can spring from the same origin in my opinion.
This does answer the possibility of sharing past life memories, colocation or multiple simultaneous memories.
The sparks just go back to the collective at the end of our lifetime here and they completely integrate with it. Every new life is a new spark but coming from the combined experiences of all lives before. How they find their way .. same thing as with the astral cord, I suppose. They just never completely loose their connection to the whole.
As for the opposites .. not sure if we do choose our incarnations, at the least we, down here (the little sparks), don't to my knowledge. We're sent where the soul or something else finds it necessary/funny (depending on your view of fate

) so sometimes that throws you into exact opposites to gather completely new experiences or to take a break from it all, who knows.
Personally I'm not that opposite to my memories, the environment just is.
Sparks staying intact and hopping from one life directly to the other? That's a possibility.
Heck everything is possible, these are just personal theories that can't be proven nor denied. As long as they work for you it's all good.
