is it possible to do One-off-Payments without the customer having a Subscription agreement?

No. You can use One-off-Payments either during signup for a Subscription (like pay-now and allow merchant to make subscription payments each month) and you can use One-off-payments on customers that already have MobilePay to an existing Subscription (e.g. customer pays automatically each month but buy additional stuff in your “” environment).
Note that MP users approves each one-off payment
Merchant can capture a reserved payment asap. There is no need to wait for the confirmation about Reserved payment.