Also, still to clarify further:

By combining the following two features:
- Pushing specific routing table changes to clients, server side.
- Using a feature in the SoftEther client that prevents such routing changes to be applied locally.

... I can in fact make sure that, when connected to my home VPN, routing changes only get used by the iPhone, so non-iPhone clients behave as desired, and the iPhone does reach the Internet - but ONLY through my VPN Server ->LAN -> Home ISP, which is what I'd like to avoid in the first place.