A civilian bartender (contractor) on a Starfleet exploration vessel in peacetime is fine. A civilian bartender on a battleship in the middle of a war is ridiculous.
See, I disagree with you here.
If anything, there's MORE of a reason for her to be on a battleship. With all that stress, mayhem and carnage they're inflicting and being inflicted? All the more reason to wind down after a shift, blow off steam, have a drink and maybe listen to Guinan's positive spin.
2 (and my main point) : Tasha was a strong enough character to decide for herself to go back and help the C. there was no need at all for the mystic "you don't belong here" mumbo jumbo. She could have maybe felt out of place herself and decided to go with the C because of that, or she could simply have done it because she was a brave and noble person. Having Guinan sense something wrong, decide what it was, and tell Tasha, was ham-handed storytelling, spoon-feeding the plot to us poor stupid viewers. We're smarter than that, and Tasha was a better character than that.
That's an interesting point. I love Yesterday's Enterprise. It was *the* episode that got me hooked on TNG and made me a fan ever since.