More fiber, more soaked ground flax, more veggies causes stool to be firmer, requiring more liquid. So what's the ratio to keep stool soft?

He's 92.  His doctor recommended it.  I haven't heard about "lazy bowel" with Miralax, but know laxatives can cause this.  My Mother has the same problems as yours - I feel for you.  It is stressful.

Try Miralax.  It's the only thing that helps my Dad.  He eats well balanced meals and has a dose of Miralax every two or three days.

Flax seeds contain soluble fibre, that is why if you mix ground Fax seeds with water you get the slimy gel texture. Soluble fibre is great for absorbing excess water. Wheat bran is insoluble fibre, it does not absorb water.

We need a combination of both types of fibre in our diet for healthy guts. But everyone's gut is different and it can take some exploring to determine what works best for each person. Some people may find they get gassy and bloating from too much fibre or eating foods that take longer to breakdown and digest.

Keep in mind it can take several days for food to travel through a person's digestive system, so a meal eaten today may not exit until much later in the week. Or if a person has IBS, it may exit in a few hours. So you may have given Mum Flax seeds last week, but when did you notice the change in her stool and has the change been consistent for several days? Did you continue to give her the ground seeds?

Foods that for many are binding, such as cheese, can have the opposite effect in someone with IBS, with explosive results. I am looking at you baked Brie.

With fruit are you giving it to her raw or cooked? Cooked fruit maybe easier for her to digest, but will still have fibre and lots of moisture. Mum swears by canned pears for dessert to have things moving in the morning. My stepdad who is in hospital, loves applesauce, but he could not safely eat an apple now.

The same applies to vegies, are they raw or cooked? Easy to digest or more challenging like beans or cabbage?

To give an example, my friend has gut issues too. She eats a banana a day, me, I can only eat 1/2 and it cannot be too ripe, or it causes me much upset. She cannot have any nuts or seeds, I have a breakfast full of nuts and seeds every day. She eats wheat, I limit the amount I consume or I have trouble. She consumes Dairy and eggs, I cannot eat eggs, nor Dairy, but can eat Goat or Ewe's milk cheeses. We are all so different, it is hard to say what will work for any one person.

Good luck finding the combination of foods that give your Mum the nutrition she needs while keeping her bowels healthy.

Everything in moderation.

What caused you to go this way? Foods are usually the culprit. Coffee and apple juice caused Mom to have loose bowels so I cut them out. Bananas and cheese can help but too much can cause constipation.

Unless you have reason to believe she is becoming impacted stop obsessing about her bowels (believe me I've BTDT), everything sounds fine to me. Constipation is passing very hard stool very infrequently, usually accompanied by a high amount of distress, and normal can vary from very soft to very firm and from multiple times a day to twice a week.
Try adding a probiotic to boost her gut flora. And the occasional laxative if things aren't happening after 4 or 5 days is nothing to shy away from.