sorry for the delay, I wanted to try this for myself before I replied. The Trinket does not have a USB to serial converter, so you won’t see a serial port popping up in the IDE, that’s totally normal.
As you have found out though, it’s incredibly difficult to switch it into bootloader mode using a recent Mac. In my case it worked without a hitch from an older MacBook (a mid-2012 model to be exact) but not a more recent MacBook Air. It’s not just a matter of USB 3 vs 2, both of those boosts USB 3 ports.
Out of desperation I must admit I decided to try the USB/Ethernet hub (USB powered) that we sell in the shop since that’s all I had at hand, and it worked first time (and thereafter) from the MacBook Air.
… not sure that helps much, but in case you want to try that this is the product in question:
but the lowdown is that hooking up the Trinket via a (non-USB powered preferably) Hub is your best bet to get it going.