Second row seat feature, will they be forward/backwards adjustable?

Nobody knows for sure because Tesla has not shown the final configuration. I think it is likely that there will be some ability to adjust the second row. We had this in a minivan years ago and it was very handy.

However, I doubt you will be able to push it back far. In the concept, the second row had three seats and the third row had two seats because the interior narrows for the rear wheels, and it seems this arrangement will likely carry forward to the production model.

Maybe the seats will be flexible enough you can take out the centermost seat and then push the other two back; but I wouldn't base my vehicle-buying plans on that until Tesla reveals the car.

In the originally unveiled prototype I think it was Elon Musk himself who showed how the second row seat moved backwards and forwards (to enable easier access to third).

That, of course, like everything with the seating can have changed since. But judging by what I remember of the unveil, they seemed to make a point of the second row moving, so I would expect it to on some level do that.

The prototype they showed had 3 individually adjustable seats in the 2nd row. Elon said the final thing will be better than the prototype, so hopefully that feature will be carried over.

EDIT: Added pic

Without the third row, would a cargo barrier with some sort of parcel shelf or retractable cargo cover take its place?

I can't imagine that it would just leave an empty area behind the second row inaccessible from the trunk.