Bathing in advance is permissible even with no unusual circumstances, and in your situation would be warranted. If possible, you should prepare in the early afternoon rather than the morning.

You are not expected – or, in fact, permitted – to put yourself in a dangerous situation in order to prepare for immersion. When you bathe at home, you should ensure that your husband is present and the door to the bathroom unlocked. When your husband is not around, a long shower is sufficient as preparation for immersion.

While the water for mikveh preparation needs to be warm, it does not have to be extremely hot. You might consider experimenting with the water temperature until you find one that is comfortable and does not lead to the dizziness you describe. Furthermore, warm water is needed specifically for washing the folds of your body and for your hair. Therefore, it would be quite acceptable to wash your hair and body folds with warm water waiting in a separate bowl or pot, while you sit in a lukewarm bath. As long as you wash the rest of your body well, the lukewarm water will be fine.