Why can’t I acces the Mac’s webserver from the iPhone using the Mac’s ad-hoc WiFi

My guess is that the iPhone doesn't consider the connection valid because there's no DHCP service assigning it an IP or perhaps gateway allowing it access to the internet.

Try running a DHCP server on your laptop or enabling network sharing even if the laptop isn't on the net.

Yes - you need to be connected to the internet when you first enable Home Sharing, but once each device has a SSL cert or whatever is stored internally, the devices will find each other and share as long as the local network is allowing the mDNS / Bonjour traffic so the devices can discover each other and match keys stored internally to identify which account is enabled for sharing.