Setting Up a Multisite QA Environment

Good morning, everyone. I'm hoping to get some advice on the most user-friendly way to set up a multisite QA environment. Since I don't develop plugins/themes, my real goal is protecting my clients against problems that can be introduced when I'm making changes to my production environment (e.g., installing/upgrading/modifying themes, plugins and scripts). I want to make sure I test every change thoroughly before promoting it to production.

Up until now, I've made these changes on the fly to live sites. I've been lucky so far, but with each new client, this approach becomes more and more risky.

Here are some of my concerns (in no particular order):

1. Minimizing Variables - The development environment needs to mirror the QA environment as closely as possible. I don't want to find that what works in QA doesn't work in production.

2. Mirroring Content - I'd like the content in QA to mirror the content in production. This should allow for more thorough testing and (in theory) make it easier to recover in case of a disaster.

3. Local vs. Web Host - Most of the tutorials I've come across recommend using a local installation for development. This seems counter intuitive (given points #1 and #2 above). A local instance would seem to add variables while making it harder to keep the QA and production environments in synch (e.g., file paths, subdomains, etc.). I was think of using a dedicated domain for QA (e.g., mydomain.net as a QA environment a production site at mydomain.com). Is this good practice? If I go the dedicated domain route, is using the same host and/or hosting account a pro or a con?

4. Security - Whatever approach I use has to be at least as secure as my production site. I don't want a vulnerability on the QA site expose my production sites or computer to threatss.

5. User-Friendly Workflow - I don't mind creating a little more work for myself to greatly reduce my risk, but I do mind creating a lot more work for myself in order to reduce my risk slightly.

I'd love to hear any recommendations you guys have regarding setting up a QA/development environment. Feel free to be as detailed as possible.