Write to somfishdev@gmail.com. One person reads that address, usually within a day or two.
It is fixed much faster with: your iPhone model, the iOS version, the app version (Settings → About) and what you did just before it went wrong.
On your iPhone, and nowhere else. There is no account and no server holding a copy. See the privacy policy for the details.
Open the subscription screen and tap «Restore purchases». Use the same Apple ID you bought with.
Subscriptions are managed by Apple: iPhone Settings → your name → Subscriptions. Cancelling there stops the renewal; the app stays unlocked until the paid period ends. Your data is not deleted when a subscription ends.
No. Everything you recorded stays on the device. Subscribe again and it is all where you left it. It is deleted only when you delete it, or when you delete the app.
Yes. On the first screen tap «See the example» — a complete portfolio you can walk through without paying and without an account.
Settings → Reset inside the app, or «Delete my data» on the subscription screen. Deleting the app removes its database as well.