There is a fanon explanation that the variations on the new Romana (i.e. the ones not played by Lalla Ward) were temporal projections, similar to Cho Je (a projection of the abbot K’anpo Rinpoche) in Planet of the Spiders, or the Watcher, a “interim” future Doctor, from Logopolis. Perhaps the TARDIS helped with that.
Okay, except there was a throwaway line by Romana during that episode where she was saying things about tailoring her new (Lalla Ward’s) body (e.g. shorten the arms a bit) that cannot be explained that way.
As for that whole scene about her regeneration, I’ve always thought that if Mary Tamm could not make an appearance, Destiny of the Daleks should’ve opened with the Doctor yelling to an unseen Romana offstage, “I TOLD you not to eat those mushrooms, didn’t I?”
Then Romana could come in, and the Doctor could appear stunned. “Princess Astra?! What are you doing here?” And the argument could commence about wearing copies of bodies. Only it’s too late for Romana to change.
Well, that part of it can be explained by the “first 15 hours” rule from The Christmas Invasion, where Doctor No. Ten grows a new hand after the Sycorax leader chops it off.
My recon for the half-human bit is that the 8th Doctor, being in a state of post-regenerative confusion, got a precognitive glimpse of 10.5 who is half human.
I would consider myself a casual Who fan. I liked the original run when I was a kid but pretty much stopped watching after Peter Davison.
I’ve watched (and mostly enjoyed) every episode of the reboot - once. So I certainly don’t have the entire mythos committed to memory, nor can I knowledgeably debate canon and continuity under various showrunners.
But what I’ve gleaned from this conversation is that each showrunner, and even each writer, seems to have his or her own vision of who and what the Doctor really is, these visions are often at odds with one another, and nobody seems terribly concerned about closely adhering to what has come before.
So while you can count me among those who have been confused by the whole timeless child bit and don’t much care for it, I’m not too worried that it will ultimately affect my enjoyment of the show, and it’ll probably all be undone by the next guy anyway.