She should be on canned food = more water, less waste to pass, less calories by volume, easier to meal feed.
She should be off all the fiber. The laxatives only work with sufficient water intake, otherwise they just make things worse. In a 12 year old cat I doubt she is taking in enough water.
Start with 6-8 ounces daily of any canned food she will eat that doesn't cause any GI upset. Add to the canned food 1/4-1/2 cup water to make gravy. Divide this amount in to 2-3 feedings. Weigh her every week. It is more detrimental to her to lose weight rapidly than it is to be fat. Your goal should be to reach her ideal weight in 6-12 months. Adjust quantities up or down to do that.
As for her hair loss:
Think food allergy...
When cats itch it is caused by fleas 50% of the time, food 40% ofthe time, and all other allergies and problems ( inhalant allergy, mites,bacterial infection, etc) 10% of the time.
Fleas and food must be eliminated before thinking of tests foreverything else.
Purchase a flea comb and comb ALL cats daily for 5 days lookingfor live fleas or the black flecks of flea dirt that looks like ground pepper.You cannot rule out fleas in cats by just looking under the hair. You must combthem. If you see any evidence of fleas on anybody....treat everyone.
Any of the prescription once-a -month products (Frontline,Advantage, Revolution) are effective. Remember though that they all work great for 3weeks, well for four weeks, they do not work for 5 weeks. Treat all cats onceevery three weeks for at least 2 treatments. Do not waste your money on overthe counter products, they do not work. Comb regularly after that and treat as indicated.
In the case of flea allergy, it is not the individual flea bitethat they are scratching. When a fleaallergic cat is bitten by a flea, they have a reaction that causes them to reacteverywhere.
To eliminate food as a possible source of the itching, the cat must go off all the things itnow eats or has ever eaten. Allcat foods, canned and dry, all table foods, and all treats. Read all theingredients on the labels. Anything that is in any of those foods the cat cannothave during the trial. If there is chicken or poultry meal, it cannot havechicken. If there is corn, it cannot have corn. If there is fish, it cannothave fish.
For the 3-4 weeks the cat is going to be on the food trial, it isoften easiest to use cooked meats, lunch meat, or strained meat baby foods. Youcan use whatever meat is NOT in your cat's current diet. Feed that exclusivelyfor 3-4 weeks. If all signs resolve, we will need to find a more balancedmaintenance diet.
To be effective you must be religious about this so that food as asource of the allergy is eliminated.
While you're doing the above you can explore the house andenvironment for anything your cat could be breathing in that it could beallergic to: XXXXX XXXXX the litter, potpourri, cleaners, carpet fresheners,plants in the house, fireplace or cigarette smoke, etc, These should beeliminated also.
Just a rule of thumb....flea allergy and inhalant allergies arevery responsive to steroids (cortisone), food allergy is not.
Take her off the prednisolone before she becomes diabetic too.
I don't think it's that simple. If her bowels adjusted to the increased fiber and you changed her to the high protein low fiber, it stands to reason that she would stool less because there is less waste (fiber). There is no advantage to using the low residue (EN) except that your other cat is on it and that might be more convenient. They are also two different manufacturers so maybe your vet thought she may handle that diet better.
Her issue is she needs to be on fewer calories. You can accomplish that by feeding less quantity of her regular food or by feeding the same quantity of a lower calorie food (high fiber or high water). Most important though is that these new foods do not create other GI upsets. That's why it is good to have a variety to try is she has problems.