Is there a way to setup an ad-hoc network without "losing" the device? Right now, when setting up an ad-hoc network, using the "Create Network" feature in OS X/macOS, the device creating the network is "lost" – meaning that it can't be used for normal internet connectivity anymore. 

My question would be, if it's possible through some CLI utility or an app to create an ad-hoc network, but still be able to use the Wi-Fi to connect to the Internet?
In the early days of 802.11b, you could set a radio to either be in Access Point -> Station mode, or AdHoc, but not both simultaneously.  And, depending on the vendor, client devices, such as PCMCIA cards, couldn't be AP's without special firmware or software.

Nowadays, operating systems and chipset vendors have blurred the lines between infrastructure networks and adhoc to make tethering and home sharing easier.  Nevertheless, just as you can't connect to more than one SSID (wireless network) at a time, you can't host an SSID as an AP and join another one as a client.  Also, most client devices (computers, phones, and tablets) don't have the radio hardware necessary to be a repeater.

To do what you want, you'd need two WiFi adapters, one integrated and one USB, or an external device that acts as a WiFi-Ethernet bridge.