Is it okay to have stragglers in your party join you in line at the front gate before the park opens?
No. Either everyone is in line at the same time or you all wait to get inside. It's your responsibility to take care of things like bathroom breaks, getting babies/toddlers situated into their strollers, getting bags packed and ready, etc. Unless you have a serious emergency (aka period, diarrhea, some kind of medical condition), I don't see an excuse when poor planing was the real reason.
Is it okay for Mom shop on Main Street while the rest of us line up for Rope Drop, and then have her join us a few minutes early?
Nope. Again, same concept as the first question. Plan your day accordingly so you don't have to cut in line while other people follow etiquette. Tell mom to take some time to shop mid-day or even late morning. Those are pretty good times to shop since not a lot of people will be in the stores before the parks close.
Is it okay for some of your group to line up early for Frozen and then save seats front and center for others much further back in the line? (Isn't this exactly the same thing as holding places in any other line?)
It is the same, yes, and I think it's pretty rude. If you can't all be there at the same time or you're only saving one seat, which isn't that big of a deal, I think, then you need to wait until all of your party is there. If you're saving 3+ seats, then you need to wait until everyone's ready to go into the theater/attraction.
Is it okay for one or two people in your party to leave the line for the bathroom and then return and join the rest of them? How about to go get churros for the group?
Again, this is a matter of prioritizing. Does your party
need churros right then and there in line? Can you get the churros before, then all line up? What about waiting half an hour to an hour later? It's all about poor planning. I can understand bathroom emergencies, baby bottom blow-outs and whatnot. These things happen. But if you've got someone pushing through a line carrying 4 churros, 2 drinks, and a Mickey pretzel with nacho cheese sauce then it's going to piss some people off.
Imagine this: let's say one or two people from each party did that, or even half the guests. You'd have so many people cutting in line, going back and forth, screwing up ride parties. People would have to step back to the side to wait for the rest of their party to board or get split up. The amount of time people have to board some rides is seconds. The CMs have no time to wait for all of your party to make it. It just doesn't make sense to me why someone would make such a silly decision to inconvenience people just because suddenly they had a craving for a churro while they were standing in line. If you know you're going to be standing in line for a while then maybe consider getting a snack
beforehand. And it's not like they don't have an excuse. The wait times are posted on the boards, the rides themselves, and the app.