Don't be afraid of knocking holes in your drywall. It's easy to repair. I'd suggest you cut a piece out large enough that it spans two studs, so when you're done, it's easy to screw it back in and clean up the cracks. Repairing a large crack isn't much harder than a small one.
It's a tempting proposition but I may just avoid the trouble. Plus, I have to figure out how to power the valve since there's no outlets on that side of the garage - I'd probably want to have an electrician come in to wire it as it doesn't seem like a very trivial job - I'd have to do a lot of knocking drywall, drilling holes, and fishing wire around as the nearest source is probably 50ft away with a corner to deal with.
If you're installing a device in the wall that is electrical and a plumbing valve... I'd suggest you leave an access panel to get to it.
You probably don't want to hear this, but I might actually re-think the whole leak detection thing. How much is it going to cost for the valve and all the sensors?
For the most part, I think IOT is a really bad idea. Simpler is more often better. It's likely this will be hooked up for the next 20+ years. It sounds like it will create a reverse path into your home network (if you're able to shut your water off from your mobile phone.) How long do you suppose the manufacturer will be supplying bug fixes? Are you going to diligently track CERT and mitre.org for vulnerabilities on this thing?
Your most likely leaks:
* water heater - if plumbed up to current code, this should be mitigated. Leaks will be caught and diverted outside
* refrigerator/washing machine - put steel braided hoses on there and replace them every 5-10 years. If really paranoid, you can get a dumb, non IOT shutoff for them for the same cost or less.