I believe that you're right. A friend of mine who worked at Apple when the Portable debuted told me the same thing, and it makes sense.
As to your other questions, I don't have solid answers, unfortunately. I don't know for certain how the LCD hardware is partitioned between the display itself and the logic board in the 100. My guess (and it is only that), based on typical practice (as well as the 100's Portable heritage), is that you could replace the passive display with an active one without having to touch the logic board. One might further speculate that using an active matrix display from a Portable could be a straightforward operation.
And your question about possibly being able to use a 180's ram module in the 100 is an interesting one. Even if the module were electrically compatible (a fact yet to be determined), though, I worry that it wouldn't fit in the tiny space allotted in the 100. I have a broken 180 that I've been meaning to crack open "one of these days," and your question might just be what motivates me to pop it open sooner than later.