Hi John,
(1) How far back spirits began incarnating in human bodies, etc.?--
I don't know if 100,000 years is claimed specifically by anyone. I did recall that a person reported that they had past lives that went back somewhere close to that far, though the earliest lives were on a different world in a different type of body. I'm a bit doubtful about that type of thing myself. In any case, I do not know when we (as spirits) began incarnating in human bodies, though I have a lot of theories. However, it is also hard to answer your question because I think of all conscious life as having a spirit of some type. So, to me, even my cats and dog have spirits (though not the same type that incarnate as humans). So, if there was something human walking around, I would assume that it was conscious and had a spirit like ours. I suppose it is possible that such a being might exist with an animal type spirit . . . maybe.
The latter is an interesting question. Someone (Ritter I think) recently compared the body/spirit problem to hardware and software. So, advanced software (such as an advanced spirit) requires advanced hardware (i.e., a sophisticated brain and nervous system) in order to work and manifest properly. This is not a bad analogy. I suppose from that standpoint, that you could have less advanced software that could still work on advanced hardware. So, possibly a more primitive animal-type spirit could operate out of a human type body, though I think it would only be capable of using a small amount of what the human nervous system was capable of. If it was an animalistic spirit, I assume it might be a bit more clever, but still animalistic. Still, I'm just guessing on all of this.
To really get down to what I myself consider to be physical evidence of when human type spirits were in human type bodies, I go to something that you see a lot of in your line or work: Art. Animals create shelters, use primitive tools, and have ways of communicating with each other. However, I don't believe they create Art in the way human's do and think of the term. So, I would say that human spirits were in human bodies at least as early as the earliest definite human art works:
https://en.wikipedia.org/wiki/Cave_painting
I'm not altogether sure of the dates given on the art shown in that article (which ranged in different examples from around 20,000 years old to more than 60,000 years old), as radiometric dating techniques always rely on certain assumptions that may be open to question. However, I definitely agree that the art shown is the product of a human spirit or something very close to it.
(2) Who do I think had more to gain .. them or us?--
I'm not sure I understand the question because I am not sure who is "them" and who is "us". I believe I am a spirit who inhabits human physical bodies while manifesting in the physical on this planet. So, I assume "them" refers to our physical bodies and "us" refers to us as spirits. In that regard, I don't ask whether my body gains from being used by me any more than I ask whether my clothes or my car gains from being used by me. Also, like my clothes and car, I believe that human bodies were made to be used by advanced spirits of our type (and sometimes by even higher spirits when they choose to manifest at this level). However, I suppose that human bodies considered as biological machines definitely gain from our usage, as we propagate, shelter, feed and protect them (and have done so in ever increasing numbers and style) since we first were embodied in them.
Cordially,
S&S
PS--In the interest of honesty I have to admit that I am not a particularly good care-taker when it comes to clothes, car or body. This is an unfortunate character defect on my part and evidence of poor stewardship.