Comet trailer near catastrophe

A longer tongue increases the forces on it...

Lengthening the tongue can increase the bending moment (and the
applied stresses if the diameter and wall thickness remain the same).
But I don't think it's accurate to say that it increases the forces
acting on the tongue.

...but increases the trailer stability...

Definitely! I've never met a trailer stability problem that couldn't
be fix by adding more tongue.