Quick reply for now, I hope to come back to this later if I get time.
It does sound like a long time on scales and technical exercises which are possibly far more complex than you need at your current level of playing. You could adjust the amount of time you spend on these, at least for a while. They are important, but no-one ever goes to a concert to hear scales

, so perhaps pieces are a more enjoyable focus as your exam approaches.
Practise methods
Try to decide if your difficulty is a case of knowing the notes, or a technical problem. I don't know the Kabalevsky piece, and can't find my copy to look right now, but if the thirds are in one hand, it could be that tension in the hand, wrist and forearm is causing the problem. Or that the fingering, although consistent, is not the most helpful one for your particular hand in that passage. this is where your teacher is invaluable (I am working on the assumption that you have a teacher, if not the best tip I could give you is to get one). Your teacher should be able to spot what the problem is, e.g. if you can play it slowly, what changes when you go faster?
Pieces: I always start from the very beginning and proceed in order and have never tried any other ways. Should I?
Say for example you always go wrong at bar 17 in a piece. First look at bar 17 and sort out what happens in that bar. Then go back to bar 16 and practise
the approach to the bit that always go wrong. This is nearly always where the heart of the problem actually is. Then when you can link those two bars together, go back to bar 15 and start from there. This idea of working backwards from the problem, tackles the matters of preparation for a difficulty, and really make you learn the piece from the inside out. As you start from further back from the problem, you are rehearsing the mental preparation required for that bit. You might be thinking of keeping the arm relaxed, or knowing that the chord you need is a D major, or that you need to put the 2nd finger on the lowest note, some sort of specific instruction to yourself to manage that passage.
You could also try starting the piece from the beginning and even in the middle of the various phrases. This again means really having to know it in better detail. If you play from beginning to end each time "mechanical memory" can take over, whereas if you can dot about and come in and out from different places, you will be getting a deeper knowledge of the piece.
Another useful technique is to record yourself playing, and then listen to the recording, noting any areas of problems or stumbles. This allows you to listen more objectively than you can when actually playing, and can make you aware of stumbles or wrong notes you may have glossed over before.
Practice: I am a home worker and lucky enough to be able to practise at least 2 hrs a day (fitting my practice sessions at diff. times of the day
Make sure you take plenty of breaks in your practise, and try not to spend hours on any one bit of work at a time. This can lead to tension, and I find breaking up the work into smaller chunks refreshes my concentration. 40 minutes of a piece is better spent in two lots of 20 minutes with maybe some sight reading in between, or a cup of tea
I think one of the most frustrating aspects of practise is that as musically minded people, we want to hear the piece as a musical experience, especially when we have a rough idea of how it shoud go, but we have to do all the nuts and bolts work on it before being able to produce it satisfactorily. We have to delay our own musical satisfaction from ourselves as our "audience" for a long time, and concentrate on the production of the various demands of the piece. Only as it becomes really competent do we get the musical satisfaction we are looking for.
Good luck with it all, and do talk to your teacher about this. There's no substitute for that one to one attention to swiftly sort out difficulties.