Is it possible to make instant coffee of varying roasts? I've never seen instant coffees being sold consisting of varying roasts (i.e. light/mild, medium, dark etc.) does such a thing exist? Or is there a reason why they haven't been tried?
There is no reason why instant coffee couldn't be roasted differently. It's made from real coffee beans after they have been roasted and ground. Instant coffee is just dehydrated coffee basically.

I think the reason you have never seen different roasts types is because instant coffee is made more for convenience. It's supposed to be quick, easy coffee. It's made for the "Amateur coffee drinker"(If there is such a thing). For that reason those purchasing the coffee probably don't care much about how it will taste and therefor care very little about the roast type.