Different releases change not just the rootfs, but also the other partitions. Generally speaking you cannot mix a major older release of the other partitions on eMMC with the newer rootfs. SD card would not solve that (there are exceptions for patch releases, for example, R28.2 with an R28.2.1 rootfs, but this isn’t what you are doing). If you can run on eMMC I’d suggest you just stick to that unless you can afford to lose the TX2.
The TX2 requires recovery mode to flash. This turns the Jetson into a custom USB device which the driver package (running on the host) understands. The other USB ports do not work for this purpose. Once the o/s is gone, then recovery mode is the only method of access.
You could in theory buy another carrier board and flash with that, and if you want to use the carrier with the broken micro-OTG connector, then just put the Jetson back on that carrier. That can be an expensive way to go, but it gives you safety for future use as well. If you were to buy a new dev kit, then both the old and new TX2 could be flashed on the new carrier board, but the old carrier would still work for non-flash purposes. The risk of simply manually installing without the flash scripts and the recovery mode is rather high.
The SD card running the current release is just a way to use dd to do the equivalent of flash, but this is not trivial and is very risky. It might work, but without experience doing this using signed content this is far more likely to fail than anyone should be comfortable with.