One thing to keep in mind, is that the Traveller is actually not all that long. Its OAL WITHOUT the tire carrier, is like 191in. Its actually not that long.
Thats smaller than the Vanilla Toyota Highlander (ie, not the newer larger Grand Highlander), which has an overall length of ~195in. And the Highlander is not exactly known for its rear seat legroom, at 28in.
The Tesla Model Y is also available with a third row, and is a bit smaller than the the size of the Traveler, at 187in. It has a third row, with 26.5in of legroom. And AFAIK, its one of the smallest vehicles in the US, with a third row.
For reference, this is what the rear seat looks like in the Model Y.
This guy is 6'3" (unfair, I know). I don't think the middle seat is all the way back either. But he does have his wife who is 5'3" who also jumps in there.
View attachment 10008
This one shows a couple of kids. Ranging from 4ft tall exactly, up through 5'4" and 5'8".
Here is the 4ft tall kid, behind the middle row. Plenty of space for the kid. But 5'8" kid hit the ceiling with his head, and back of seats with his knees, at the same time (I know the headroom statement isn't relevant to the traveler, but just saying, its a small space).
View attachment 10009
So, for a useable third row, I think that the Travelall in the future will much preferred. But, I could be on board with some "emergency jump seats" for the traveller. As mentioned, they could work for small kids (maybe 10yrs old or younger). But I'd worry about any actual adults fitting in the vehicle.
I've actually got a friend with a Model Y 7 seater (they just got it). I may ask to sit in it the next time I see them, just to see how it fits, for giggles.
For context, my 7yr old is about 4'4" last I checked, and my 10yr old is like 4'9", and I'd be worried about them fitting in the model Y size backseat for much longer. Our current vehicle has 32in of rear legroom, which is more than enough for them, but the Model Y is a solid 7.5in less legroom than what we have.
One potential complication, is that the underfloor storage area of the Traveler, is likely already pretty heavily utilized. Remember there is the harvester (the gas engine), the spare tire, already supposed to be in there somewhere. So I wonder if there would even be enough space to "eat into the floor" space to have them fold down.