I dunno about how the metabolic system works in regards to HIIT. Having said this I've started slowly cutting again to bring out definition. In 4 weeks I've lost about 3 kilos with no change in strength - in fact I'm still increasing my lifts - and my abs are coming out. Like you said there's no such thing as spot reduction of fat. The only solution is to reduce your overall body fat percentage to a point where you're happy with the 'look'.

Anyway when it comes to burning fat, remember is 80% what you eat and 20% training. Hell if you diet right you don't even need to train to lose weight. However this is inadvisable given the potential for lost gainz. And also all the other cardiovascular/diabetic/health risks associated with a sedentary lifestyle. HIIT/marathon running/cycling. You don't really need to overthink the metabolic factors behind this. Even if there is any difference - it's honestly not going to be that much. Just get at least 20-30 minutes of cardio per day at 80% max heart rate and you'll be fine.

The first thing you wanna do is get some information on your body composition. Your best bet is a DEXA scan. However an alternative is to view images of people at different body fat percentages and see which one closely represents you. Stuff like this http://www.myfitfuel.in/mffblog/wp-content/uploads/2016/04/body-fat-percentage-men-e1460747675286.jpg is useful in that regard.

Now the next step is to calculate your lean mass by subtracting the estimated body fat percentage. At the start of my cut