That doesn't sound good. Field engagements like this are always stressful. There are unfortunately some people in the field that like to be able to place blame on others, if the project fails it is your fault if it is successful it is because of them. If you feel that this next project isn't going to go well say you will only go if the test plan is changed. I would also ask for some kind of compensation. The field generally makes more money because of the stress and issues that arise with field engagements. Ask if there are any spots available on the Mexico trip coming up that would be a nice reward
That sounds rough! I would have a serious talk with your boss and anyone else you need to talk to voicing your concerns about next month. I understand not feeling comfortable presenting something that you did not prepare, especially when you think it will be a failure, and you can't do it yourself because you have your own job to focus on. That other office really needs to get it together!