I got the
upside-down spray bottle at Lee Valley, but I'm sure others sell them, too.
You apply the BT after every rain; can't remember frequency if you're not dealing with at least weekly rain (the bottle will tell you). We water once a week or so (as needed) when it doesn't rain, so that translates to once-a-week BT spraying for us, roughly. If it's actively raining for a few days in a row, I wait until the rain is done. I rarely see active moths when it's off-and-on raining.
The BT comes as a concentrate that you mix with water. The little bottle will last you many seasons, at least it does for me and I'm spraying 50 to 60 plants very thoroughly.
This year I'm going to do row cover and BT side by side and see which is less hassle. One might think "row cover, of course" but we have so many weeds out here that I have to remove covers and cloches too often for my liking. We'll see.
ETA because I forgot the last question.
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Cabbage moths don't bother the other crops you mentioned. They have their own pests, which differ due to location and climate. We don't have any pests that bother tomatoes here; hornworms would be your enemy if you live where they are. I think Heather mentioned that she's used BT for them. I have no idea if it's the same BT as for cabbage worms or a different one; check the label in the store. I get masses of flea beetles whenever I plant lettuce in my garden; I use nasturtiums as a trap crop – the only companion planting scheme that has undeniably worked for me. The flea beetles prefer the nasturtiums (where you can kill them off if you like) and leave the lettuce alone very nearly completely. We have no pests here that bother any of the other crops you listed. In affected areas, you'll have cucumber beetles going after cucumbers and squash borers going after melons and squash. I can't offer experienced advice against them, though.