Does the resistor value on D+ and D- line matter on Arduinos?
What happens if there is no resistor or the resistor value is really high (over 10k)?

The USB connection won't work. Remember that there are pulldown resistors in the hub/host with about 20k ohms.
The 5.1K value for R8 and R9 on the Qwiic Pro is a slight mistake in the schematic - these resistors use the same footprint as others but should have a different ohms value. The vendor has omitted the BOM (bill of materials) for this product, so we would need to measure on a finished PCB for the actual value - which I'm pretty sure is 22 ohms.