HIIT training will be way difficult every day. Let alone a couple of days. Do the fast but just keep to some strength training and some light cardio. Maybe once you’re used to doing extended fast you can then introduce some HIIT training and see how you go. You’re setting yourself up for failure otherwise.
Your TDE is around 2500 calories (estimated). A pound of fat is 3500 calories. Without exercise you will probably lose about 3/4 of fat for each day fasting, although the first day is very little fat because you are burning up carb reserves. I would estimate you would lose 4-5 lbs of fat over 7 days.
Don’t do HIIT and extended fasting. Terrible idea. Especially for a first time faster. Learning to balance electrolytes is enough of a challenge, HIIT would just make it more complicated.
Walking is the best exercise IMO while fasting for extended periods.
The absolute best way to calculate it would be to do it, afterwards eat clean for several days to let the water weight return, and then compare measurements and weights for before and after. Trying to do it before is called counting your chickens before they hatch.
I know it’s fun to play the numbers, but in the end you just need to get through it and then check your results. With everyone’s body being so different, it’s hard for anyone to give you exact numbers other than roughly half a pound a day of fat and a lot of water weight. But have fun, I just finished a 4 day fast and it was amazing.