While true that single-channel test gateways are troublesome and certainly not compliant, some do support downlinks (and the screenshot shows this one does too), and hence also support OTAA.
Yes, before the OTAA Join is started. (When the join is not explicitly started in your code, then the first uplink will make LMIC first trigger the join, so then anywhere around the end of setup will do.)
You could try larger values, like MAX_CLOCK_ERROR * 20 / 100. After the OTAA Join succeeds, you’ll have to limit the channels to the one supported by your single-channel test gateway, as an OTAA Join Accept in EU868 will include network settings that (rightfully so) expect a proper gateway. See Can LMIC 1.6 be set up to use a single channel and SF? - #11 by arjanvanb
So, all said, @Verkehrsrot is right.