" Wouldn't it be helpful to have plastic insert containers with handles
so she can do all the planting comfortably on the kitchen counter and
then just carry them to the yard and pop them into the cement planters
window boxes?"

So you think your 85-year-old mum will be able to go out to the garden, remove the inserts, carry them into the kitchen, do the planting, and then carry them back out to "pop them" back into place?

A rolling stool (search Google shopping) might be good for her, but ask her if she wants one. Maybe the kneeling and standing is keeping her knees in good shape.

Hi Lauren, it sounds as though your mom is doing quite well. What appears to be uncomfortable to you is likely to be positions that she finds works well for her. My mom (84 yrs old) still does all of her yard work except for mowing.

There is a large selection of inserts already available but I do agree with lazy_gardens that it might be a bit challenging for your mom to carry large filled inserts and swap them out and the rolling bench would be an excellent gift.

Use small plastic pots (quart size/4 inch) that can be easily lifted out as needed to change the plants. It's commonly done in planting window boxes and larger planters with filler to cover non-soil areas.

If your mom has physical issues that make gardening a challenge, here is an article with tips the two of you might find helpful so she can continue to enjoy gardening.

Gardening with Physical Limitations