Pearmability Tuning Vs Cap tuning for MW AM
Selectivity was marginal at the top of the band despite using cascode
  JFET amplifier circuits that preserve existing tuned circuit Q.


That is untrue - if you only alter C (or L for that matter) the Q of the circuit will vary with the square root of the ratio of L to C (or C to L depending on what type of tuned circuit you used). Q will definitely not remain constant across the band of frequencies.


  How would a TRF radio go with slug tuning?


With difficulty I would estimate - keeping them all ganged to produce the same filter frequency would be a problem.


  How would tracking go if one attempted to use the existing osc coil in
  a common 3 slug setup?


TRF receivers DO NOT use an oscillator coil (or an oscillator): -



They just use banks of tuned filters (operated in parallel) as per the above diagram.


  Would the gain be higher at the bottom end of the band?


The selectivity will be greater at higher frequencies and less at lower frequencies.


  How do the tolerances of inductive slugs compare to 3 gang variable
  caps?


Well, for most "vintage MW AM car radios" the slug was mainly used to retune an oscillator because the radio was invariably a superheterodyne type with a fixed intermediate frequency of 455 kHz. This means the tolerance is immaterial but the mechanical movement that pushed and locked the slug was the weakest link (usually).