1. Q1 I could not find the path from RF N divider to Ftest/LD in the LMX2485 data sheet
block diagram, but this path exists as you mention MUX[3:0]=15?
Q2 Customer’s goal is, Input FinRF=50~1.3GHz, get digital output from Ftest =FinRF/384.
Use LMX2581 just as divider. Backward of LMX is MPU.
Anyway, the value which customer can select for RF_N is, 31~1023 Or 55~2039 in data sheet
P26 Table15/Table 16? Is there minimum/mximum frequency for Ftest/LD pin?

2. MB15F72UL has this
Path in the Block Diagram