Thanks for writing me with your question. I’m sorry to hear that you are having such difficulties. I should start by saying that I can’t be certain about what’s going on with your voice without hearing an example. But a few possibilities come to mind.
The exercises you mention may be fine or not depending on how you are doing them. Based on the problems you are having I suspect that you have gotten in the habit of doing them in an unproductive way. Stretching is good, but if you stretch in a way the eliminates a good resonating form you will have trouble. (An example of this would be stretching the front of the mouth or just the face and not including a stretch of the back of the mouth.) This is because the throat space will close, which includes a raising of the position of the larynx. When this condition exists it often leads to sore throats because the tissue of the throat gets irritated. The inflamed tissue leads people to wrongly assume that allergies are to blame.
The closing of the resonance space and the higher larynx position would also make sense with the difficulty and cracking in the higher range. The extension of the range is dependent on the stability of the larynx so the vocal folds can stretch for the higher pitch. If the larynx is allowed to rise it will not allow the folds inside to stretch for pitch. The high range also depends on the stable larynx to keep the resonator of the pharynx open. The proper resonance reinforces the vibration and amplifies the tone so the vocal folds can vibrate smaller in order to vibrate faster for higher pitch. When this is lacking the voice gives out or strains in order to produce higher pitches.
Like I said, I can only give educated guesses based on your descriptions. If you want more accurate feedback you are welcome to send me a recording. It can be some exercises or a song. But if you can show me some examples of the difficulty I can give you a more accurate assessment.