NP-completeness and difficulty of all possible solutions Take the SAT problem (or any other NP-complete).
Is the set of all possible combinations of SAT problems are NP-complete?
Or maybe there are combinations that can be quickly solved?
If I take a random logical formula of n variables is always always be NP-complete problem? There are no exceptions for any particular forms of formulas?

Not all subsets of SAT are NP-complete.  Schaefer's dichotomy theorem identifies infinite subsets of SAT that are in P, with the remainder being NP-complete.  The known subsets that are in P are 2-SAT, HORN-SAT, ANTIHORN-SAT, XOR-SAT and SAT instances that can be satisfied with all-true or all-false assignments.
