I'm looking for disposable underwear with minimum absorbency. Recommendations on brands or where I can find some?

If you don't mind washing them there are several cotton reusable incontinence panties available. I have seen them on Amazon even. But if you want disposables have you tried the newer "slim" or "sleek" type? I know Depends makes some, I'm not sure about other brands but they are designed to be worn under more fitted pants for instance. Also I found that sizing down in size on the regular daily Depends made them much more comfortable for Mom, she too was feeling they were bulky before I tried that. Now she wears the nighttime super absorbent at night because the daytime version doesn't hold enough and then the daytime during waking hours. I think they have a choice of absorbency, though I get them in bulk now for her from Costco because they are so much more affordable that way. I have the nighttime type set up on auto delivery with Prime for the same reason. I'm not trying to sell the Depends, I really think fit and preference varies with each person, it's such an individual thing and the brand that feels good for Mom may not be the same as it is for your mom so there is some trial and error to find the one that'd just right. Good luck!

What about the new Period Panties that are on the market? They are expensive, but washable and get good reviews.

Apparently you can wear them all day during your period and not leak. They may be just the ticket for your mother.

There are some pull-up briefs that have a partly enclosed pouch for a pad. You could use thinner panty liners or thicker pads, and they wouldn't be so likely to be removed. The briefs are often advertised here on mail-order brochures aimed at the elderly. The brief fabric of the pouch would get wet, but if the pad was changed regularly the smell would have less time to develop. Could be worth a try.

Have you tried the incontinence pads? Tena and Always Discreet both make a variety of lengths and absorbancies and are commonly available.