We are going thru similar situation with my elderly mom's cat Susan who is also 17. The ulcerated mouth, difficulty eating & pawing at mouth, grinding noise, weight loss is the same.
Have blood tests been done? Susan's blood tests have revealed kidney problems (very high Urea and Createnine). Vet says ulcerated mouth links up with high Urea result and likelihood that ulcers elsewhere in her body
Susan currently on a drip at vet to rehydrate and flush out toxins that have built up due to failing kidneys, pain killers, anti biotics ..... I rang vet this morning and Susan ate a little overnight and seems brighter. Brother just rang to say he happened to be passing vet and has dropped in to see how she is too and I'll be speaking to vet later this afternoon to see how things are.
The only thing we care about is Susan's quality of life. Best possible scenario, bearing in mind awful blood results, is that she has a few months. More likely she only has a few weeks unless we are prepared to bring her in every few weeks so vet can put her on a drip and we won't be doing that.
Not sure whether any of this helps you decide how to go but am sending best wishes for Tipsy and you