Charging a QC 3.0 USB-C Thunderbolt specced laptop via generic 15V charger
The question is essentially this: If I boldly apply a plain 15V/5A source to a device designed to QuickCharge 3.0 proprietary specifications, will my device charge?

The answer is: likely not. The QC process starts with the default 5V level, and then device request higher profiles by manipulating with D+/D- wires and setting certain DC levels. This is a sequential process, and if the initial handshake fails, the device will likely not engage higher charging rates and consume more power from charger.

The second question is, will the bold 15V5A fry my QC3.0 input circuitry? The answer is "who knows", unless the device datasheet explicitly says that it is tolerant to any voltage up to 20V regardless of QC presence, or the schematics of the charger section inside your device is known, with all related datasheets.

You have made the right choice to use 5V only, please continue to run it in this mode for your safety and safety of your device. However, in all likelihood, your device will take only 500 mA and charge very slowly, if at all.