Will a VA Disability make someone ineligible to attend ROTC?

No.
VA disability does not relate to fitness for duty. You conditions may make you unfit but the fact you are rated by the VA will not.

Very possible, since the intended result of ROTC to to get you in the service as an Officer. Whether its the Reserves or AD, you still will have to meet service requirements

It wouldn't DQ you by default. But during the medical screening any illness/injury you have may fail requirements. I would recommend speaking to an ROTC advisor for more guidance.

It can be done, but the person will have to be found fit for duty.
All VA disability ratings will be considered case closed and upgraded to rehabilitated or discontinued.
You can't be on active duty or reserve/guard status and be VA rated at the same time.