Navigating the ship was the job of Geordi, then Wesley, then a succession of minor guest characters and extras like Rager, Allenby, etc. Data's post was operations manager, the same job Harry Kim held on Voyager. He also functioned as de facto science officer, and was third in command after the captain and first officer -- which is what being a second officer means.
Tactical and security are two different jobs which are conflated onscreen for convenience. Security deals with internal crises and crew/away-team safety; tactical deals with external threats and space combat.
Initially, Ranul Keru handled both security and tactical duties aboard Titan, per Starfleet convention. But after his injury, he had to be on reduced duty and Tuvok took over his tactical responsibilities. Keru is recovered now, but it's still a worthwhile division of labor.
For what it's worth, my issue is less about what Data's job was on Enterprise (ops! knew I didn't have that right) or Keru's or anyone else's - it's more the fact that it's only in recent books that I feel a big deal has been made out of 'second officers' (whereas I'd say it's more important what their actual 'job' is, like Data at ops). I would bet I could go back and reread Q-Squared without seeing a single reference to who is the second officer of Enterprise - yet, it's a very prevalent (and yet, to me unimportant) fact that keeps getting brought up in recent books. But whatever, I can live with it.
She never actually stopped being faithful. The thing with Leybenzon never went beyond flirtation, and it lasted mere weeks before the events of Before Dishonor scuttled it for good. And since they worked things out, everything's been fine between her and Vicenzo, as far as I can recall.
This all depends upon your definition of 'faithful', but I'll go so far as to say that it wasn't just the incident with Leybenzon in BD (which you very handily cleaned up in GttS by the way - in one of the few books I've been 'ok' with Miranda), there's an incident in Gods of Night where she thinks someone is flirting with her on the elevator and she goes to great lengths to say how she's happily married, only to be embarrassed because she wasn't being flirted with (except, then I think we get in the mind of the male from that scene, and I think he was, but whatever) - now perhaps that scene was meant to reinforce the idea that she IS faithful and the Leybenzon incident (which was really just an author error - I love Peter David's work, but an egg is an egg) was just a minor slip - but instead this new scene reinforced my feeling that she's not particularly 'faithful' in that she's much more concerned with people flirting with her than in doing her job. Is it her fault someone's flirting with her? No. But as a married man, my awareness of someone flirting with me is no more than, 'uh, yeah that's nice, whatever, I'm not going to even dignify this with an answer' whereas her response seems more of a case of 'thou doth protest too much'.
That said, to be honest, none of that has ever really been at the heart of my problem with Miranda. I haven't liked her since she first appeared in this relaunch (Resistance? Q&A?), and that was before the 'incident' above. Part of it is that I don't agree with her life choice. She's choosing to leave behind her family of husband, 1-year-old twins, and a 5-year-old so that she can be second officer on the Enterprise. Now, I've got a 2-year-old and a 4-year-old, and my wife worked for the first year of my first son's life before decided she's rather be home - that said, I'm not a 'the woman should be home taking care of the kid' kind of guy. She chose to do that, and I'd have supported her if she wanted to continue to work - and many women can and do do that, but it requires balance. Miranda's choice shows no balance to me. It's all about 'her'.
Let me give you an example. Let's say I was offered the position of Vice President of my company, which means a huge salary, but I had to relocate to Iraq. My wife and kids would never go to Iraq. And the position is indefinate, as long as I want to be VP of this division, that's where I'll be with maybe a 'once a year' trip home, and maybe daily calls to my family. Have I just described Miranda's situation? I think I have, and I'd have NEVER chosen her way. I wouldn't miss my kids growing up, not even for a lot of money. Some people would do that, and Miranda is apparently one of them. Frankly, her choice of this is even worse than what I've described above, because for large sums of money I could at least understand the difficulty - Miranda is essentially choosing this for prestige. Come on, it's presitigious to serve as second officer on the Enterprise E. It's like someone giving up their family to be on American Idol. It rubs me wrong.
Is part of this her husband's fault? I don't remember any more - was he given the choice of joining her on the Enterprise and he turned it down? Then perhaps there's more of a problem in this marriage than mere flirting - because I think there should be some compromise going on here. Could Miranda have found a Starfleet posting closer to home? What's her goal here - indefinate second officer on the flagship, barely ever seeing her kids? Captaincy of her own ship, so she can essentially do the same?
I don't understand her, and I don't like her - but that's ok. She's well written, and I'm ok with the idea that a character on the Enterprise is someone I don't really like. This isn't the first Trek character I haven't liked, won't be the last.