What are the fundamental beliefs of Ahmadis?

No, I don't believe Ahmadis are muslims because they reject the Aqeedah that is to believe in the finality of Prophet Muhammad(SAW). You've asked me yourself this question and I replied you with honesty.
You're right that Kufar and non-muslim are not always the same things.
leaving prayer is another thing and according to majority of Ulema it doesn't make a person kafir but rejecting it outright does make one kafir.

You can try but there's no chance of me accepting Mirza Ghulam Ahmed as prophet, his rude language is enough to put off any sensible person.