Yes. You can shoot video at whatever ISO you want. But it's like shooting stills at ISO 80, 64, or 50 when the native ISO is 100 - it's not recommended for best quality. You will notice that when setting ISO, some settings will have a line above and below them (as mentioned in the video I linked to). These are 'Extended Settings'.

You're very lucky. You really need to follow instructions to the letter when doing firmware updates as it is easy to 'brick' your camera if you do it incorrectly.

I thought I'd write up what happened to me to help others with similar predicaments. I stupidly didn't read the instructions when I updated the firmware on my a6700. I inserted a freshly a6700 formatted SD card with the firmware update. And proceeded to start the update process. When the screen went black for a few seconds, I thought there was an error and shut off the camera, not realizing this was part of the update process. (I think it's a dumb idea to have a black screen for the update process.) My camera wouldn't turn on. All I'd get was a black screen and a blinking red light on the bottom left side of the camera. I tried all sorts of things for hours upon hours. None of them worked. Here's what did work: I erased the SD card and formatted it in exFAT format on my Mac. Then I added the firmware update. Then I inserted the card and turned the camera on. After a few minutes, the screen turned on. (I think this should be done with a full battery.) Anyways, I thought I'd have to have my camera repaired. But it turned out to be a simple solution.