when I click on edit, I have this error message
So where is this link you're clicking on, in your index template? And what is the name of your form-handling file? My guess is that that you've just got the URL wrong in the hyperlink referencing the form-handling file.
As for your second question ... look-and-feel is modifiable for individual subsites, not individual packages. We do things this way because commonly people want to give a site a uniform look-and-feel and this simplifies the amount of customization needed to accomplish this.
So what you'd need to do to change the look-and-feel of your testing package is to mount another instance of acs-subsite in the site-map, give that acs-subsite instance its own master template, then mount your test package under the new acs-subsite instance.
This assumes you write your package in the generalized style we use. If your package is customized for a particular website and will never be used outside that particular website there's no reason you can't define a master template in your package then refer to it directly in the "master" tag within your various content pages.
