essential = absolutely necessary
___________
plus any modifiers = plus modifiers, if there are any (i.e., there don't have to be additional modifiers)
___________
The
be, in the form
being, has been omitted. This is common.
Their future being more secure, ...
___________
The use of
for in your example is wrong. Let's change it to meet our needs for an example to illustrate the point of your question.
You are late, and you are unprepared.
This has two main clauses, as occurs when a coordinating conjunction joins clauses.
that, which, if, unless, when, although, since, because, etc. are
subordinating words. Therefore none of the following are main
clauses:
that you answered my letter
which I was about to say
if you were there
unless you can come earlier
although he may be very intelligent
because I didn't know how to do it
CJ