I've been to 3 GTO shows (two in the Europe tour last year, and the London show in June), and managed to meet her and the band every time, but not always organised. The UK shows last August/September (apart from London, due to the venue), she did organised meet and greets. She didn't come out in London but we managed to quickly say hi and be hugged and kissed by the tourbus before she left. The show in June this year, the venue told everyone to leave pretty much straight away, so we did, but about 10 minutes later Amanda jut popped out the front, spent about 20-30 minutes with the 15-20 people that were around, and then proceeded to party on the rooftop.
I guess it's mostly down to the venue and whether they're strict with kicking people out or not.
As for soundcheck, last year's London show she turned up about 2pm-ish? but this year it was probably about 4 or 5 I think. However, she had been doing press all day, and the soundcheck ended up running over, cos she was late. But whether she stays to chat when she turns up will depend on how much time she has, and also how many people are around.
I guess my best advice is to just go for it. Turn up early. If you get to meet Amanda and the band, then great. If you don't, then at least you'll be at the front of the queue and have an awesome spot :3 and then hang around afterwards - if she doesn't have time to stay, then c'est la vie, but you'll never know unless you try
Hope this is helpful in some way!