You can hook up the chandelier to the existing outlet box (make sure it's one for a hanging fixture), and then put a few hooks in the ceiling. That way you can move the chandelier when you change the size of the table.

Use a white wire, white chain, and white hooks, to blend in with the ceiling.

Can you swing your table 90* so that it runs parallel to the window and therefore more centred under the light?

I have the same Dilemma as you and what I did is I expand my table and put in the center the chandelier. What I am thinking to do next is reduce my table and leave in the center of my chandelier. I am crossing my finger that idea works. I have a very long table 120+. I have to buy a tablecloth 140 inches long then I have to cut 10 inches and it fit perfect. I wish you a good luck in your decision I know is not easy but just use your imagination to see the whole picture done.

Can you put more pictures to see how long or wide is your dining room?.