Welcome to the forum! If you're up for it, you can also introduce yourself and your project further in the Social forum.
An answer to your questions:
1. it depends on the conditions. There's composting from industrial scale to home composting. The material is of course completely natural and worms like mycelium . I would say that a few months is a good bet. I'd be very interested if someone else has more reliable input though. We haven't tested it ourselves.
2. it doesn't mind some drops or short exposure to water. It does mind longer exposure, also coupled with wind or lots of light etc. In general, harsh conditions, and especially water, make it biodegrade. Especially when the outer protective layer of mycelium is breached, since that layer is mainly what makes it kind of hydrophobic.
Hope this helps! Will you document the project and share it afterwards?