Let me try to explain why I personally saw her as an inexperienced captain rather than a veteran as you have ably demonstrated to be the case.
Obviously I'm aware that being thrown tens of thousands of light years away from home is not that likely and therefore any commanding officer will be out of their depth, but Janeway did not seem, IMO, to know what she was doing. Even from Janeway's first appearance meeting with Paris, we learned she was a "best and the brightest" officer, or else Admiral Paris would not have chosen her for his staff on the Al Batani, but I didn't get the impression that she was a veteran captain. To me, she appeared to be a science officer with leadership skills given command of a starship, and the best episodes were where her science skills took centre stage.
She grew as a captain over the seven years, and after Seven joined the crew we saw her as a mentor, as Tuvok had supposedly been to her. Which brings me to my main point. I think the information given in "Revulsion" was a retcon since there was nothing else canonically to tell us she was captain prior to being given command of
Voyager.
Christopher, you say in "Prime Factors" that it was established that Tuvok served under Janeway for several years prior to the show, but all that could mean was that she was an executive officer and he was security/tactical, he would still have been serving under her. According to Memory Alpha, Janeway met Tuvok in 2356 after her first command when he dressed her down in front of three admirals:
Janeway first met Tuvok in 2356. Janeway was dressed down by Tuvok in front of three Starfleet admirals for failing to observe proper tactical procedures during her first command. Although the incident bruised her "Human ego" at the time, she ultimately realized Tuvok was correct. (VOY: "Fury", "Revulsion")
Her age here would be somewhere between 18 and 28 based on the wildly differing birthdates given so her first command can be assumed to be when she was in first command of a mission, rather than a starship.
During the mid-2360s, Tuvok was temporarily assigned to Jupiter Station where he often wrote to Janeway. (VOY: "Tuvix") From 2365 onward, Janeway and Tuvok became close friends, and Janeway found she could always "rely on his insightful and unfailingly logical advice".
When Janeway was assigned to the
Billings as a command officer (no canonical evidence to suggest she was the captain) she may have asked the captain to assign Tuvok as security/tactical officer in 2365, or Tuvok was assigned to the
Billings and suggested Janeway as executive officer.
Either way, nothing indicates that she was a captain before taking command of
Voyager, the possible "Revulsion" retcon notwithstanding. I believe Tuvok's mentoring of Janeway was established to aid in her mentoring of Seven rather than for any other reason.