1. Totally fine to mix cereal with whatever she'll eat! You get more of a nutrition boost if you use formula, but if she won't eat it, no big deal.
2. It really depends on your DD how long you take to work up to a tablespoon. Both my kids were completely uninterested in solids until they were old enough for finger food. Your best bet is to follow her cues. If the cereal isn't causing tummy issues, her formula intake isn't dropping, and she's willing to eat more, go for it!
3. IMHO, I really don't think it's a huge deal to be consistent. #2 was all over the place with naps and feedings, plus I had another kid to tend to, so it was almost impossible to be consistent with her solids. I tried to at least stay consistent with what meal she ate, but sometimes it was 30 min after bf, sometime 1 hr, and sometimes it got skipped. I don't think it mattered.
4. I wouldn't worry too much about her getting bored with cereal, and since she's younger, it might be a good idea to take it slow. Get through all the kinds of cereal first and then move on to some of the milder first foods, like avocado or pear.