"Суть" действительно может быть глаголом, но в современном русском в таком виде никогда не используется. В викисловаре есть пример: "И дальние родственники суть наши ближние". В этом примере "суть" означает "быть/являются" (to be во множественном числе, то есть are). "И дальние родственники - наши ближние". Но я крайне не рекомендую использовать "суть" как глагол - так никто уже не говорит, и многие вас не поймут. Как использовать? Заменяем "are" на "суть". "Difficulties are part of life". "Трудности суть часть жизни". Причина редактирования: исправил пример использования согласно замечанию Риты.

Simple "не важно", is more common:))))) Especially when you don't want to discuss something.

There is a fairly common spoken "не суть важно", or "это не суть важно". A fancy version of "doesn't matter".

Yes. But it is bookish:))) In Russian it is not used for ages, but until 19th century educated people knew Church Slavonic langauge and it affected Russian a lot. You can find it sometimes in certain books: philosophy, math, religion. Some people can use it in speech when in the mood to speak in this style. Of these people some know that it plural, some do not.. Some are affected by the meaning of the noun суть and would use it when they want to say "X are (or is) essentially Y". Some just use it to mean "are". We do not PERCEIVE it as a verb. Some of us KNOW this, for some others it is just a "strange word" that used at this position. But we don't perceive it as a verb. Maybe the most enthusiastic users of it do, but I do not:/ I use it rarely. I know that it is 3d person plural but -уть doesn't sound like 3p ending for me and the word as a verb. Apart of philosophy, math and religion, logic: when you are teaching logic and you need to say "all A are X" you do not say "все А Х" (it sounds not good:/) and you do not say "все А есть X". You say "все А суть Х".

Синонимы слова "суть" это "главное, основное, сущьность" (и еще много). In English - main thing, basic, essence...