5:2 done right is actually well within the means of reasonable, if done correctly. I wouldn't consider it antithesis. It's 22% deficit on average, but with so many days eating at reset basically, the hormones and stress are saved.
Really eating at TDEE on 5 days for what you do on those days, 2 days eating at 25% of that day's TDEE.
If your workouts are pretty much the same, then doing a weekly TDEE basis for daily average could provide decent figures too.

Some just take the stated numbers and use them (2400/600), but their own workout level makes them actually higher (3000 / 750 say).
Or they still try to restrict on the 5 days and make those diet days too, which is wrong.

So that could have been one issue. Also when your workouts are could be another. Lifting and HIIT done correctly are basically the same type of workout, all out effort, tear down muscle, repair in 24-36 hrs. If your 25% eating level was the next day, you ruined the repair aspect probably.

The 2 days should be separate and non-workout days, not back to back.
The 2 days would be best served not following an intense workout day that requires long recovery and repair.
The 2 days could follow an easy cardio workout probably without a problem.

1 - strength and endurance should suffer when you go in to a diet if you were already doing it and can compare. First response of body is to store less carbs in muscles, that's attached to water - first big water weight drop of weight.
With less carbs in muscles, harder to do big focus carb burning workouts.
2 - Your preference to help you adhere. But 24-36 hrs after a lifting style workout is when body needs fuel to do repair. So timing could be great or awful for that. Many have done an 8 hr window no problem, actually smaller. Depends on if you start the window with a snack just prior to a workout.
3 - Depends on what you plan to do. Since you are eating at maintenance 5 days out of the week, you likely won't gain any water weight back, that happens the day after the 25% eating level day.
So likely going in to a diet everyday will actually cause the first big water weight flush again.

Yep, I still wouldn't do the HIIT until you've gotten some carbs in to ya though, just so you have the energy to hit the HIIT hard. Like lifting, if not done at maximal effort, not actually getting the benefit from it.