I would, and also do on my gateways, actually have a slight offset for TIME_BETWEEN, one 50, one 60 and a third one 70, so that it might not happen that they all only scan at the very same time, but so that some almost constant scanning is implemented, with SCAN_DUR (10) and TRACKER_TIMEOUT (360) being the same on all. This is just some personal preference though and keeping TIME_BETWEEN (60) for all yours will also be fine.
As DISCOVERY should only every be on when you actually want to discovery new devices you have, it doesn’t really matter which gateway, OMG or TGW, does the discovery, OMG actually has a functionality that it turns discovery off again after 30 minutes, which helps to not have passerby or neighbours’ devices discovered by mistake, where with TGW you will have to turn DISCOVERY off yourself again.
HASS_DISCOVERY however is just an additional functionality to DISCOVERY in that it makes sure that the discovery messages are in the Home Assiatant format. Turned off it was compatible with the discovery in openHAB. As this is not necessary any longer with recent openHAB changes the HASS_DISCOVERY setting will be removed soon.
The latest OpenMQTTGateway 1.8.x releases already have an internal tracker sync implemented, so that a tracker is only getting an offline message when the very last gateway has lost contact. With the next release of Theengs Gateway this will also be implemented there so that your 1 Theengs Gateway and 2 OpenMQTTGateways setting will not have such occurrences of unwanted AWAY states any longer.