"ERROR Connection Timeout" on gatt.connect()

This is what I get

If the address is reported as the same then I'm surprised it's an issue - it's actually the exact address string you see there that is used to initiate the connection, so if it's the same there should be no difference.

I can see 5 to 10s between packets

This would probably explain some connection issues too - I forget the timeout on connections but it's probably no more than 5 seconds. If it's reporting 7ms intervals but is sending only 5 sec, that's probably upset Espruino's bluetooth stack a bit too!

If the interval is reliably 5 seconds, you might get more reliable connections by doing requestDevice, waiting 4.5 sec once you get the response, and then initiating a connection.

Could the cause of the closed connection also be on the Mac OS side

Maybe. If you're trashing WiFi as well it can definitely make things flakey but usually it's quite good. If it happens again and the LEDs on the MDBT42Q breakout flash then it'll be a reboot caused by an error reported from the BLE stack though (in which case the error should be reported on Serial), in which case it's something I could hopefully track down.

I only have one loaner device from the company

I guess they might loan one out to me.

Good luck with the testing - however it looks from the device address string that it may not be the problem you were seeing. However you should now be able to connect to your phone at least!