I use a bedside commode which she urinates in & empty the urine downstairs. She needs commode for feces. Can I use the commode liner for bm?

The commode liners are easy to use and keep the plastic bucket from absorbing the body waste odors.
Eventually you will not be able to rid the plastic of those smells.

Nothing is easier than the commode liners. No need to use all the kitchen trash bags fir the bedside commode.

I get mom's from Amazon and everything goes in them.

I love the idea of cat litter though and will try that too.

Use cat litter in the plastic garbage bag placed inside the commode pail.

The cat litter will clump whatever lands in it, and can be thrown away as garbage. It can also help absorb the odors. Use a kitchen sized plastic garbage bag as the liner, and remove and replace it after every use.

I would place a small plastic grocery bag in the receptacle for the commode I used for my Husband. Most times though I would just empty the waste into the toilet. Much easier than either trying to dump the contents of a bag into the toilet or closing it up and putting it with the garbage. But both were done at various times.
If it was easier to dump into the toilet I would put an inch or two of water in the basin so the contents would come out easier and the basin easier to clean.

Bottom line...whatever is easier for you is going to be the better way for YOU. If liners are going to make it easier for you to clean up then use liners. If there is a risk of spilling when you have to go downstairs then use liners.

When my husband was bedridden, and had to use the bedside commode to poop in, I just put about 6 inches of water in it before he sat to poop. I also sprayed some Poo-Pourri in the water, as my husband was in our living room in a hospital bed, and I didn't want my house smelling like poop. Then all I had to do was empty it in the toilet and flush it down. I of course wiped it down afterward, so it was ready for the next day.
I've never used the liners, but think they would be more messy than just putting some water in her bedside commode.