I'm not a believer in: "I'll go start waiting in line while you do something else."
The entire party should be together before getting in a line. If some want to rush, and others don't, then either some in the party have to adapt to another style, or the group should divide and be OK not riding together.
Of course, lines are long and kids are unpredictable, so I totally understand if a group got in line together but someone has to leave the line for a restroom break. That shouldn't mean the entire group - who may have been waiting together for 30+ minutes - needs to leave the line. But that's really the only exception I can think of. If someone has an issue that makes them unable to wait in a line, that's what the
DAS is for.
(Also, I'm not talking about QS restaurants. It doesn't matter if one person or the entire party is in line because most of the time, one person will be putting in the order for the entire group/family. I've never seen someone hold a place in a QS line for a group that all place separate orders.)
Here's the one I hate: one person (usually younger kid) rushes into a line, other guests file in behind, and then the parents/other siblings (stragglers) show up a minute later. But now there are several or more people between the stragglers and their other party member. Often, the stragglers will call the kid to the back of the line with them. But
sometimes, the stragglers think it's OK to pass the people in front of them to catch up to their kid. Annoying.
As for your specific scenarios:
Is it okay to have stragglers in your party join you in line at the front gate before the park opens?
I'm an early person. I'll get up early to get into the park. I tell anyone with me who is not willing to get up early to text me when they are in the park. "OK, but what about if we're all up, and one person stops to pick up Starbucks. Can't they rejoin us at the gate?" Sure. And if they aren't that far behind, and the line hasn't grown much behind you, then it's not the end of the world. But having one person wait the entire time, and having an entire group join them just before opening is annoying.
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?
It's a lot more like a loose mob at rope drop, and not so much a dedicated line, so I think you have more wiggle room. But let's see if any frequent rope droppers have a thought on this.
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?)
You are right. This seems less bad for some reason, but is basically the same thing as holding a place in line. Specifically, the times I've seem Frozen, there's always plenty of good seating so I've never seen a situation where this would really be that much of an issue.
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?
Always OK to leave the line for the bathroom. And for the churro scenario, I'm a lot more forgiving if the group entered the line together and have been waiting. If there's an opportunity for someone to jump out for a moment, grab a churro, and then get back in line, I wouldn't be upset about that like I would if the churro-buyer had not entered the line with the rest of the party. Having said that, I'm imagining a scenario where it's easy to hop in and out of the line - not a scenario that requires physically walking back and forth through the entire queue.
At the end of the day, it's the people who take advantage of this that are the problem, not the majority of people who legitimately "just this one time" have to cut through the line to catch up to their party.