Hi,
From what I’ve learned and re-read, excessive water drinking or hyperdipsia can certainly lead to hyponatremia (or low sodium) as you were thinking. Drinking too much water without any other medical disorder would likely result in euvolemic hyponatremia of moderate severity which can give you symptoms of anorexia, nausea, muscle cramps, lethargy and confusion. Unlikely to lead to seizures or coma unless ingested very rapidly.

Generally, healthy adult kidneys can process 15 liters of water per day if you drink the water in small quantities throughout the day. Not that it’s recommended to drink that much obviously, more like 2.8 L (3 quarts) for the average adult which, taking into consideration the water content in food, ends up being that proverbial 8 to 12 eight ounce glasses of water. The rate at which you drink is also important (about 240 ml (or 8 oz) per hour) so that your body can absorb and also excrete the water as required.

Not that this information helps with the constipation which I sympathize with you about. You’ve done the fiber and fluids, exercise etc. Then from a conventional point of view, you’re looking at stool softeners, lubricants like mineral oil (which you have to be careful about due to risk of aspiration in neuro disorders), stimulant and osmotic laxatives, all of which I’m sure you don’t want to be on long-term. I’m sure there must be another answer in other health practices that I’m not familiar with and hopefully someone more knowledgeable in this area. Hope you find an answer!