Sigil of Force + of the Night, and dual-wielding

As you noted, currently weapon stats (weapon damage, weapon stats, sigils) do not apply when using a weapon kit. Each weapon kit has its own weapon damage (scaled according to level).

However, ArenaNet has stated they are working on making sigils and weapon stats work with weapon kits.

From playing with a dual dagger elementalist, I believe that sigils which proc 'on critical hit' are tied to the weapons skills, so you're correct in assuming that the life stealing will only proc when using the first three skills.

Correction: According to this thread: http://www.guildwars2guru.com/topic/43566-weapon-sigil-questions/ Critical Hit sigils should fire regardless on the skill used, even utility skills, so as long as the weapon is equipped, the effects should be active.

I haven't used the 'on critical hit' sigils too much, as I tend to rely on the 'on weapon swap' sigils, so I could be wrong.