As long as your newly purchased prepaid SIM card is not registered, no mobile network services will be available (except emergency calls).
For existing prepaid SIM cards, all mobile network services remain available. But from 01 September 2019 they cannot be topped up anymore and so there will be no possibility to extend the validity of your non-registered prepaid SIM card.
Yes, emergency calls are possible even from a non-registered prepaid SIM card.
Yes, any initial credit will only be available once your prepaid SIM card has been activated. In order to activate a prepaid SIM card, you must first register it.
The procedure to be followed in each case can be found at the following links:
Prepaid cards can only be registered in the name of natural persons.
Please find information about that at the website of your provider or contact the customer service.
As long as your newly purchased prepaid SIM card is not registered, no mobile network services will be available (except emergency calls). That affects roaming services as well.
Roaming services for existing prepaid SIM cards remain available if your tariff plan provides them and enough credit is available.
But from 01 September 2019 non-registered prepaid SIM cards cannot be topped up anymore.
If an internet connection (e.g. WiFi) is available, you can use the online-registration outside of Austria as well.