Many questions, so let me answer the core ones.
A gateway is uniquely identified by the network id and the gateway id. So there is a TTNv2 gateway with ID eui-0011223344556677, and there is a separate gateway on eu1.thethings.network (V3 eu) with ID eui-0011223344556677. These two are currently handled separately. At some point we can try and find a way to either copy old coverage data over, or merge the two. We can do this because we know TTNv2 and eu1.thethings.network are essentially the same network what coverage is concerned.
Raw coverage data is never deleted. Only aggregated data is hidden from the map when a gateway is offline. At least this is the case for V2. On V3 we do not have a NOC, so the only way to know if a gateway is online or offline is by looking at messages received. To start off we will likely have to assume a V3 gateway is never offline and always show it.
Mapping V3 is already possible using the Android app. It is also possible using a gps tracker and setting up a webhook integration. It’s a little tricky to do, and therefore we need a template. Issue is that the template doesn’t allow optional fields, making the “experiment” option difficult to implement. My plan is to release a template that does not have the experiment option, but one can manually add it by editing the webhook.
Edit:
I’ve filed a PR to add the webhook template: