Hi there and welcome!

As you've been eating so low for a long time it is highly likely your metabolism will be depressed so be prepared to gain initially. (Read the 'upping cals and what to expect' sticky to help you be prepared for what to expect).

That's a pretty intense training schedule so your body needs fuelling well. As you are lifting while your metabolism increases and your body considers you to be eating in surplus, you may add some muscle which is obviously good and will help raise that metabolism. There will also be some fat gain and water gain but once your metabolism has rebalanced you should start losing again.

It is a slow process and it does take commitment and time but it is so worth it. Consider that alternative which would be to eat 1200 to 1500 cals for the rest of your life.

In my opinion your training is really intense and may be unneccessary. Is this for fun, weight loss ortraining for something? If you did that kind of training eating 1200-1500 calories that isnt enough to fuel you for workouts, especially if that is a gross 1200-1500. I think you may need a reset to let your body feel assured about the food to come. Upping you calories to about 24-2500 for a few weeks could really help you out as far as energy etc. and if you havent had a diet break eating tdee for a week or more then you should really consider a reset. Your metabolism may be suppressed and just needs a break from the undereating and overtraining. I agree its a slow process in some cases but would you rather eat 24-2500 cals or 12-1500? You may gain initially from your glycogen stores being depleted and you may be eating at a surplus depending on how suppressed your metabolism is so that may cause a gain. If you would like to cut before trying a reset I'd suggest a 15% cut as you have a minimum of 15 lbs to lose. When you get within 10 lbs of your goal it should switch to a 10% cut.

So forget the HRM for your strength training, totally invalid and inflated.

Use MFP if you are still going to attempt an eat-back method.

If using the TDEE deficit method, be honest with your HRs per week.

I also see some unnecessary stress in the routine, kick boxing following leg day.
You are probably wasting most if not all your effort from the leg day, because you aren't getting chance for rest and recovery and repair.
So all you are doing is adding on additional stress to the body.

Exercise tears down, it's the rest, recovery, and repair that build back up - if you eat enough.

Stress inhibits fat and weight loss.
Frequent intense exercise is stress, so is diet.

Why intervals after the long lifting session?
Because you want the cardio improvements to lung capacity and lactate clearing and glucose usage?
Or you heard it burns more fat than just cardio?

The latter is HIIT specifically, and if you are really trying to accomplish it (doubtful after strength training anyway), you are just adding even more stress to your workouts, and really wasting your lifting effort.
HIIT is lifting, for those that only want to do cardio. Which you aren't doing. Would you lift with the same muscles day after day?
No, and you've shown having a split routine you know that. Why?

For your fears - has your routine continued to work this last year?
Beginning of weight loss you have big margin for error in doing things not great and even wrong, and still seeing results.
Depending on how bad things are done, that margin for error tightens right on up. Most see it on the last 10 lbs.

Use this to get better estimate of BMR, I'm betting you've burned off fair amount of muscle up to this point, and have more BF% than expected using other BMR's.
Then be honest with activity time, and really suggest easing up, or you'll be fighting this battle much longer than needed.

http://www.myfitnesspal.com/topics/show/961054-spreadsheet-for-bodyfat-bmr-tdee-progress-tracker

When at maintenance, and no stress of diet in place, you can have more stress from exercise with less ill effects. Though even then, you've probably heard of people over training and going backwards in progress. Too much stress. Even eating more can't balance out.

Just gotta comment on your icon, because that idea came up in training sessions all the time - "ya, but I'm seeing progress".
Well, far too many times its slow because of things not being done the best way they could be done. Only at elite levels are things slow because there is little room for improvement without everything being done exactly right.
Just like weight loss, many hear slow is better. But slow because you forced your metabolism slower from loss of muscle and other reasons is not the same thing as purposely selecting goals that makes it slow and safer.