Can you install it via an android phone?
Also ill be doing the install via the headset, stupid question but do I have too buy it direct on the headset on its web browser?
You can install it with just the headset, check the installation guide. (Trying to install to your headset using phone would just overcomplicate things.)
You can purchase it wherever is convinient. If you want to access the purchase on multiple devices, you can create an itch.io account for free.