The problems you are describing are pretty much right-on-track for only being a month post-op. She will adapt to new eating habits. The surgery is major and recovery takes a while.
You can bolace water through your mom's J tube. Just fill the huge syringe with water and push it through. It's time consuming but it helps with hydration and it's something she can do on her own.
As for her postponing the treatment, there is probably an underlying reason she doesn't want to start chemo. It might maker he sick, might not. My dad had very little nausea with the chemo. They give anti-puke meds with the chemo so you don't feel the effects as badly. She may be having some pyscological issues too but the sooner the better. My dad's cancer is apparently identical to your mom's. (Stomach, spleen and pancreas portion removed. Lymphs also involved) and I think the chemo/radiation is what put him into remission. His cancer has since returned but there are no regrets about the agressive treatment and drastic surgury.
Hang in there. It get's easier. Keep us posted.