I believe you're over thinking it (especially if you're just getting started). I would just work out when you can, and get your macros throughout the day.
IIRC the biggest problem with tap water is usually added chlorine, which has potential links to cancer. If you're concerned about this, you can simply fill a pitcher or something and sit it overnight...
Cooking for a non-cooker can seem intimidating and hard, but its really not. Just remember that, barring starting a fire, there isn't much that can go wrong. You...
If you've been running a 750 calorie deficit average, you would have to eat nearly 13,000 calories (750x14 + ~2500 maintenance) in a day to "obliterate all progress."
I've noticed this as well, especially in canned chicken. Per 56g I've seen from 8 to 12g protein. I use 10 when I calculate, and assume the difference won't make or break me.
I found that, after running a 500-1000 calorie deficit for 2 weeks, I can have a cheat day (aka binge day) and I simply cant eat enough to do significant damage to the deficit ive built up.
I'm a dad of 2 kids under 3 years old and I'm literally tired every day on about 6 hours sleep. I've been working out steadily for the past 4 weeks, making gains.
Good on you for taking the next step (arguably should have been the first step, but I don't really care how you get there as long as you do) and starting to calc your actual dietary...