How are users able to actually use features such as pinching if they don't have touchscreen monitors?
There are keyboard and mouse equivalents for most of the features, they've kept in mind that the applications need to be able to be navigated using keyboard and mouse as well. For instance, you can use mouse scrolling to replicate the pinching (to zoom).
These are documented in the Help and Support which is in the charm at the right side as well as when you press F1. I can't remember the category but it wasn't hard to reach; however, they're quite trivial.
I plan to use Windows 8 on my desktop, and I want to get the full use of any applications I download.
Yes, you will be possible to make full use of most of them, the only feature that lacks on a mouse is rotation but that shouldn't really matter as professional applications will probably include an on-screen control allowing you to rotate.
I want to ensure that if I ever release any application I develop to the windows store that there is a way for my users to get around this situation (no touchscreen for multitouch events).
By default both work in an equivalent way, you can however assign specific actions to specific events, although it would be stupid for an app developer to ignore a part of the users and explicitly make them behave different.
Have used quite some Metro apps when I tried myself and I never found an occasion that I was lacking functionality, so this will most likely rarely be a problem. And if it is, you can probably raise it to the developer that went wrong which might be happy to fix it.