I am going to attempt a response to a couple of your points.
First the tubes that you are using for the front and rear rails; check them to make certain they are straight. Just lay them on a flat surface and roll them. If they are not straight, you will know it immediately.
As for being different length - I don't believe it makes a lot of difference as long as you can lock the tubes in the front and rear rails.
If you feel like you need longer front and rear tubes, you could buy some EMT as your local hardware store to use for this. It is slightly smaller in diameter (O.D.) but works fine.
Now to align your extension table. Same procedure, slightly abbreviated, works for floating tables.
We start with the basic assumption that the Main Table has been properly aligned (especially square to the blade).
Move the headstock to the right, close to the right extension table if that is the one you want to align. Move it to the left, if that is the one you want to align.
Loosen the nuts that secure the Extension Table Base to the bottom of the table. Do not take the nuts and washers off, just loosen them.
Install the tubes into the front and rear rails and secure the the tubes into the Main Table. This is a point of reference for the rest of the Extension Table (or Floating Table) setup.
With the Extension Table and Main Table close together as shown in your photos, you should now be able to push the Extension Table tight against the Main Table and secure the thumb screws to secure it there.
Having done that, you now have to secure the Extension Table to the Extension Table Base.
Secure the washers and nuts finger tight on the studs that hold the Extension Table to the Extension Table Base. Get them all finger tight before you put a wrench on any one of them. Now snug the nuts on all four studs working around the table like you would securing the studs on your car tire. Now, once again, tighten all the nuts on all four studs.
This being done, you should have eliminated the original problem.
The tables should be the same height and square to one another.
NOTE:
These tables doe not have perfect edges (machining detail) and there may be some inconsistencies that you just have to live with.
You may also have to loosen the front and rear rails from the table and then resecure them in the same manner as we did with the Table Base.
I hope this helps.
