Does the license allow this if I don't map a domain to it and operate it as an IP address OR use an obscure term (staging etc.) in a odd domain (vs. sub-domain as mentioned in docs)?
Yes. As far as I'm aware, Pixel & Tonic consider a development site to be any non public domain.
Is this a feasible approach?
Is there a better (noob-friendly, learnable) way to do this?
Lumping these 2 together since they're directly related. This question and subsequent answers might help you out as well as Craft's own list of resources.
It really depends on what you want to learn. When you spin up a droplet or other virtual machine, there's a lot of moving parts there. You'll be dealing with a lot of server management and not necessarily Craft development. It's easy to go really deep down the rabbit role and come out with a lot of (depending on what you seek out) useless knowledge. If you're just tinkering around, you probably want it as hands-off as possible.
Instead of paying for 2 droplet instances, you could spin up a virtual machine locally, like Virtualbox or something prebuilt like Laravel's Homestead. A droplet is also on the internet, thus a "public" server, so unless you want people hacking your instance, you'll want to secure it down. That just adds another layer of complexity.
Virtual machines are great solutions but they're also overkill usually. I find the easiest approach is to develop locally and use something like MAMP and then push it to staging and/or production servers. You don't need to worry much about the nitty gritty of permissions, firewalls, etc.