Another fun fact that might affect/help your testing of downlinks transmitted by remote gateways, after the device has joined: (at least) in the EU868 frequency plan, a downlink in RX2 uses SF9, regardless what SF was used in the uplink. And that uses an increased power:
This does not apply to the OTAA Join Accept (as at that point the device does not yet know about the TTN-specific settings, which it actually gets in the data of that very Join Accept). But after joining, or when using ABP, you might turn off the close-by gateway, schedule a downlink in TTN Console, and transmit a SF11 or SF12 uplink to increase the chance that TTN will tell the remote gateway to use RX2 with SF9 and a higher transmission power for a downlink.
I’ve no idea of any of the above is useful for debugging the topic at hand. 
However, testing for RX2 is good anyway!