My FIL had also lost his ability to speak before he had difficulties swallowing so I have no idea what it felt like to him. This is usually the case
But one of the symptoms for those without cognitive decline is a feeling of something stuck in the throat which may be what your Mom is feeling. The nasolaryngoscope may give you some answers. Unless there is a tumor that the x-ray didn't pick up my bet is the CT scan will be clear. You may want to contact a speech therapist to do a swallow assessment. That is the simple non-invasive way find out what is going on. The speech therapist can also determine what food consistency will help.
To answer your question, yes it can be a result of the dementia and her food consistency may need to be adjusted to accommodate any problems she may be having. Her food may need to be "mechanical soft" which includes ground meats, soft veggies, and other foods that are easy to chew into softness. See if you can figure out what foods are giving her the most trouble since the level of difficulty seems to vary. Does she do better with a soup or ground meat than with a piece of chicken? If she is having problems with liquids, try "Thick-it", which can be purchased at your drug store, to thicken her liquids to the point they are easier to swallow. You may even need to puree her food so they are easier to swallow.
The other problem could be a GERD where the stomach contents back up into the esophagus. Again you will have to figure out the foods that cause the problem and avoid them. My husband has this problem and I know not to feed him dry breads or dry chicken (especially white meat).
There are lots of causes and hopefully the scope will give you the answer you need
You might want the neurologist to update her diagnosis if she is further along than Mild Cognitive Impairment
You are doing all you can to get to the bottom of it and I do hope you find the answers you need soon.