Hard stool that stays in the colon for more than a couple of days is called "fecal impaction." Most medical sources recommend treating it with manually removing the stool (by a doctor, if necessary) and using rectally administrated enemas:
Softening of hardened stool and stimulation of evacuation with enemas
and suppositories is often helpful...Most ...
This saccharide is of very minute risk, absolutely, and even more so if compared with glucose, fructose or saccharose.
Caries is the result of microbiota producing acids which dissolve the enamel, and
this effect is enhanced when this takes place under plaque when and where saliva cannot dilute the acids
For reason 1:
Most bacteria tested ...
This is not a medical recommendation, you should speak to your doctor.
Generally speaking, paraffin oil, lactulose syrup and dietary fiber supplements are common OTC medications for constipation. It is important to drink plenty of water (specifically with added fiber). For individual advice, talk to your doctor.