I had a similar issue with oscillation in a hammified ARR-15 several years ago.  Turns out that the little aluminum cover was missing from over the IF area tube sockets.  Once a substitute cover was made and installed, the oscillation problem was cured.  Several years ago I modified two radios with reduced IF coupling caps and they worked well, with no issues.  The math verifies that the IF bandwidth is proportional with coupling capacitor value.  The equation can be found in RDH4th, page 418, Figure 9.12.

IMHO, the ARR-15 is an under appreciated receiver.  First Collins designed receiver to cover a large portion of the HF band.  First Collins receiver using an autotune mechanical memory.  Uses the second and third PTOs designed by Ted Hunter for Collins, one for RF and another for BFO,  Both will hold a frequency setting to within 250 cycles after several autotune cycles.  The RF and IF stages are permeability tuned.  The variable IF allowed for the crystal calibrator to tune the receiver to any frequency covered by the receiver without auxiliary equipment.  

I can see the heritage between the ARR-15 and the 51J.   Everything is there in the ARR-15 except for double conversion and all was done by 1946.

Jim