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:
https://shop.pimoroni.com/products/usb-multi-function-lan-adaptor
but the lowdown is that hooking up the Trinket via a (non-USB powered preferably) Hub is your best bet to get it going.