I had a rummage online to try and find some info on directional filter design (for spectrum analysers) and the best I could come up with is in the old HP journal from August 1979. This covers the classic HP8566A spectrum analyser and it uses a directional filter on range 1 up to 2.5GHz.
https://www.hpl.hp.com/hpjournal/pdf...Fs/1979-08.pdf
Have a look at page 12 where there is a decent picture of a directional filter and also a brief technical description.
Also, your recent VNA plots of the IF1 input show a reasonably low VSWR across a 200MHz span and this suggests that the 50R resistors might be intact so this is confusing me a little. If the resistors were toasted to 2k ohm then I'd expect to see the VSWR skyrocket at the edges of that 200MHz span.
Probably best to sort out the correction table for your VNA cal kit or make an SMA cal kit using good quality end launch connectors. A carefully built cal kit like this is good to about 3GHz even with basic corrections in the cal kit definition file.