Marc Blake hangs around these very forums. I've never read his books or gone to his writing courses, but most people on here recommend them.
Regarding this dramatic ending, can't it be lifted from the sitcom, placed in a word doc entitled "future projects" and used for something else sometime?
After all, you said it's your best work and you're not ONLY going to write this sitcom, are you?
In regards to your original question, an established writer could get away with a long, closing scene with hardly any laughs, such as, for example, Delboy's poignant speech to his newborn baby in Only Fools And Horses.
But, as a new writer, different rules are applied. That's life.
Of course, saying that, it COULD work, as long as it's sharply written.
*********EDIT***********
Blimey! A page and a half of new posts were posted on this thread while I wrote this, leaving my words a little out of context now.