Is there a replacement kit for the 11851B cable set? I know the individual same length cables are available for something like $563 each.

The set is obsolete as of something like 10/3/2005.

Is there a replacement part for the longer cable?

If three shorter cables were ordered would they be "phase matched" ?

The set is obsolete as of something like 10/3/2005.

Is there a replacement part for the longer cable?

If three shorter cables were ordered would they be "phase matched" ?

Or as needed for any other performance verifications such as 85044A, 85046A, maybe 8753D can't remember offhand if its 7mm.

The 8753 A/B/C does not use error correction for verification. There is a system verification, but that is always done after error correction so phase matched cables are not important, as long as the cables are stable.