As (another) Brownie Leader, I think you're getting a bit of an unfair pasting from some. The comments about it being better for you to pull your DD out are particularly uncalled for, and very un-Guiding
The Home Contact is an established Girlguiding policy, but we've always had this as an additional safety measure. We would always take emergency details with us as well.
Young Leaders (turquoise uniform) are girls under the age 18. They cannot count as adults in the ratios, which should be 1 adult:8 Brownies outside the normal meeting place. So for 16 girls there should have been at least two Adult Leaders (navy uniform) aged 18 or over. If you don't think these ratios were met then you should certainly query this.
If they went with other groups it sounds like this may have been a District or Division trip? It may have been that other groups were told a different time, as others have said. Unfortunate but sometimes these things do happen.
We also wouldn't tick girls off as they were collected, but would acknowledge the parents and make sure we saw each girl leave.
I'm not sure if we'd have phoned if we were due back early - it would depend on how early it was looking and how long it was looking like we would be waiting.
As a Leader and volunteer, I certainly consider myself accountable to GG and to parents, I'm not some Saint beyond reproach. Policies are as much about protecting leaders as girls.
I'm sorry your DD is feeling a bit wibbly about future trips - hopefully by the time the next one comes round she'll be excited for it! I'm actually quite glad we didn't go yesterday, we've done Drayton Manor before as a group and its always far more painful than going as friends/family and have always found them slightly disappointing! The faffing, the queuing, the crowds, te endless trips to the toilet. Painful.