Somehow, I missed this thread til now- sorry, guys.
Last question first: C-420's 25 and 26 were sold to the British Columbia Railway in July 1972. You probably caught the 26 in transit, on its way to the Pacific northwest.
As far as power going beyond Maybrook, I don't think it ever happened. I seem to remember somebody (Henry Tulip?) saying that crews were informally told to keep an eye on both locomotives and cabooses while in Maybrook. The fear was if anything strayed onto the New Haven beyond Maybrook, the L&HR might never see it again (especially in later years).
(There are still a few veterans around I can ask about this to be sure- if I hear anything different, I'll post it.)