Trainlawyer--
Another nit, a support, a quibble, and an issue.
----The nit: The (all-diesel) FP-9 was 4 feet longer than a straight (freight) F-9. The FL-9 was another four feet longer again. (Roughly 50 feet, 54 feet, 58 feet.)
----Support: the E-33 were dirt cheap: after all, norfolk & Westerrn had discontinued their electric operation for operational reasons, and the only other customers potentially interested besides the New Haven were scrap dealers. I think I've seen the price-- can't remember it, but I think the New Haven managed to pick up five-year old 3300hp electrics at a unit price perhaps ?? a sixth ?? what new 1800hp diesels would have cost.
----A quibble: you point out that even electric locomotives need maintenance, which the New Haven could hardly afford (and the maintenance costs probably tend to increase with age). Still, the newest "old' electrics (EP-4 and EF-3) were "clones" of the GG-1. Since PRR/PC/Amtrak kept the GG-1 in service for another quarter century,
the maintenance that would have been needed to keep their electrical and mechanical cousins running wouldn't have been huge. ... My guess is that a more important factor was that the passenger DIESEL fleet -- Dl-109 and two maintenance heavy post-war models -- was coming due for replacement, and replacing them with locomotives that could play electric for the last few miles into Grand Central was just icing on the cake.
---Issue: I have read somewhere (?book called ???the fall of the New Haven Railroad?? ?) that Alpert gave as his reason for scrapping the older electrics that "someone had told him" that the railroad's own generating plant (Cos Cob) was worn out and could fail at any minute, despite reports by consulting engineers that it was sound. If the REAL reasons for replacing the electrics with FL-9 were the ones you describe (or, for that matter, if the real reasons were something that couldn't be admitted in public), this statement was just window-dressing and camouflage. Which, given its surface implausibility (at least as reported), it may have been.