CMs let any amount of people go catch up with their family or group in a line. On my past trip I saw CMs twice ask people as they were cutting through line if they had groups ahead and both times the people said yes and the CM said ok and smiled. The second time I saw it there were about 6 people trapsing through the line. Im not sure if they're trained to do this or whatever, but its pretty silly.
Depends on the CM. I might let one person through, maybe two. Any more than that? No way.
The issue here is that CMs are trained to give the Guests outstanding service first and foremost. The trick is to do that and still try to be fair to everyone. If a CM didn't personally see the line-cutting there's not much we can do.
Someone here tell me how to handle this problem: In our situation, I have 2 girls that are legally blind ....
We finally gave up on shows all together because it was so frustrating!
Make sure you tell the CM that your GAC clearly says you must sit in the front row of the theater (and they do). If they continue to try to direct you to the back, politely ask to speak to a manager.
I do mind people doing it, but what I mind more is the people who would come up behind me to cut would manhandle me. They couldn't just tap me on the shoulder or verbalize that they were going to cut me, no they would actually put their hands on my waist and various other places and then say they were going to cut me.
I really don't like strangers touching me, so all the grabbing was getting to me.
I had an Ex who would turn and scream at the person at the top of her lungs, "I don't know you! DO NOT TOUCH ME!"
Not surprisingly, this was quite effective.
Definitely make sure the CM knows exactly what accommodations you need... They probably just get used to sending everyone with a GAC to the same area and need to be told that your needs are very specific.
By the same token, understand that not all GACs work in the same way. I once had a family of Guests who had a GAC card allowing them to sit in the front row of a theater. When they tried to use it to bypass the line at an attraction (in this case a ride), I politely explained that it didn't apply there, and that they should join the regular queue. The reaction I got was explosive, to say the least -- with the mother of the blind child screaming at me, "She's BLIND! That's the same as a mobility impairment!"

Now, I'm not blind (Gods willing) but I'm sure there are thousands -- if not millions -- of visually disabled people who would disagree with her. It really made me feel badly for her daughter, it did.
The responsibility of watching for line cutters, then using your judgement to decided if they're meeting somebody or not, then deciding to boot them from the park - that's just too much responsibility and judgement for those front-line CM's. No offense to any CM's, but for the most part, these people are young, inexperienced people in low-paying jobs. You don't want to force these young people into making decisions that will ruin somebody's vacation.
And yet, we do each and every single day -- despite your low opinion of us.
I once had a situation where I wasn't assigned to a particular position at an attraction, so I walked around to the FastPass line to see how the wait times were looking. A young girl (about 14 or so) was trying to talk the CM at FP Return into letting her in without a FP -- she was claiming that she was trying to find her brother who was already in line. I smiled and said, "Oh, no problem. I'll be happy to walk you through the line to him." Well, we go through the FP line, all the way to the loading station -- but no luck. Well, she turns to me and asks, "Since I'm already here, can I go on?" I smiled my biggest smile, looked at her and said, "Oh, heavens no. You saw how long the wait was." The sad part was that if she hadn't lied about her brother I would have been more inclined to have accommodated her.