actually there are MANY products that use the 2.4 band that can interfere with Wireless but none do it as bad as a Microwave can.
well this is just some extra info but not specific to you other questions... if you have a 802.11 N Router and 802.11 N Laptop or something else then you will enjoy some awesome speeds BUT if you have ANY devices on the network that are B or G modes then you will lose the N compatibility. This means that if you have something that runs on G then your N hardware will drop down to G as the Router is forced to do so to compensate for the older device. The B or G or N setting means it will use the fastest it can but some devices can force it slower.
Yes but not all devices will be able to connect to the 5GHz band. This means that you will need to upgrade the hardware if it doesn't support the 5GHz band. Also if the router/modem is from your Internet Provider then there is a 99% guarantee that it won't support 5GHz at all. You will more than likely need to purchase a thirdy party router.
Channels 1-14 are based on 2.4GHz and 5GHz uses Channels 36-165 but just because you can't see those doesn't mean you can't use them. It just means that it isn't set for it...you have to go into the router admin and change it to use 5GHz instead...you can't use both so it uses 2.4 GHz by default to support as many devices as possible.
This article didn't mention a VERY important aspect to the WiFi interference...and that is Distance. The farther away the router is from the Microwave the weaker the affect is to the router. I have mine on the opposite side of the house and I never receive any interference at all.
There are many other tactics to combat this but the cheapest solution is to just move the router as far away as possible and often that works quite well. This may hurt speeds compared to N mode but if you have anything using G already then you aren't using N mode anyway.