...Yet some still think Chakotay must have been a full Commander because they never heard the "Lieutenant" part spoken out loud on screen. This despite Chakotay's predecessor, Mr Cavit, also having been a Lieutenant Commander "only".
But that's relatively clear-cut. The hairier issue is whether Tuvok was a Lieutenant or a Lieutenant Commander during the first half of Season One. His pins indicated the higher rank, and he did take part in the mutiny that apparently got Torres demoted, soon after which he started wearing the pins for the lower rank. So it would be appealing to think of him as a Lieutenant Commander originally.
But he gets called "Lieutenant" half a dozen times before his pips decrease. Half of those could be interpreted as informal "My right hand" type references rather than actual indications of rank. Alas, the other half consists of formal introductions, sometimes by Tuvok himself, in situations where either the title "Lieutenant Commander" or "Commander" would surely have been used.