Changing gear when in ERG makes no difference, as the trainer will still make you work to that same watts.
If your assigned watts are too high you can change this in Zwift. The workout session will then base the segments on this number. If you are already in a session, like you was today, there is a slider on the left of the screen that allows you to change the effort to 90% of your FTP. I have to do this sometimes if I can hit the watts towards the end of a session. If it's too easy you can also ramp it up to 110%, which I've only done a few times
Re cadences .... you don't fail a session if you don't stick to the assigned cadences; you only fail if you don't hold the correct watts. So if you are asked to hit 220 watts at say 60 cadence, but can't hold this, then just raise the cadence to something that is achievable. Similarly, if you're asked to hit say 115 cadence, but can only spin at 100 (for example), this is OK. Obviously you want to try and hit the correct cadences but you won't get penalised if you don't.
Does this help?