But the photos of the nightjet.com website show armrests between the seats. Are these armrests movable?
Yes, from the photos and train scheme it seems that those NJ carriages are old OeBB bmz type seating carriages (used to be very common for OeBB and other railway companies in the region), thus armrests definitely can be folded up and won't be a problem
In the 1970's in compartments on some trains had opposing seats that could be pulled together or nearly together. This made a makeshift bed that one could lie on. Is this possible on present day OeBB Nightjet trains?
No, the seats in these bmz carriages can be pulled toward the center a bit, but not as nearly together as it used to be possible in older carriages with similar 6 seats compartment design (AFAIR, in this region of Europe the last carriages with the chance to have the proper "makeshift bed" in the seating compartment were used by Serbian Railways (e.g. overnighter Belgrade-Budapest)