Could an eloctrolyte inbalance (potassium is high and others are borderline) be causing chest pain, headache, +waking at night because of pounding ?

NO: But something else going that causes an electrolyte imbalance could be the cause. High potassium (unless very severely elevated) is actually a good treatment for a minor arrhythmia.

May be: High potassium could be dangerous and can cause arrhythmia's . To begin with it can cause some extra beats or palpitations. Rarely it causes chest pain. Extra peat or palpitations could sometime be misinterpreted as chest pain. But you should go to pcp, get your electrolytes checked and evaluated for chest pain.