I think your body is absorbing the water just fine; did you know you can absorb up to 2 litres of water by taking a shower or hot bath? If are so concerned about how much water your are absorbing, you can increase it simply by taking a shower more than once per day.
Additionally, hard stools may be related to constipation and not water malabsorption. I wouldn’t really tie hydration with hard stools in my opinion.
Also, most people gain weight after a colonic because guess what happens after you get one? The colon gets filled back up again! So the 5 pounds of weight you gained likely went straight to the colon and you could be constipated again (hard stools)! A lot of people lose weight with colonics actually, some can lose up to 20 pounds!
I would examine your trace mineral levels because these can influence your water retention abilities as well. What do we do when it snows? We put salt on the ground and it melts the ice and absorbs the water. So you could be deficient in salt, which is made up of minerals. If you were to increase your trace minerals intake, you likely will notice a benefit in general.
One of the battles with the diet is constant dehydration so its pretty normal from my experience.
What are you doing to reduce the inflammation of your gut?