Re: はず/わけ がない または ないはず/わけ
Oh I humbly suggest you that you're missing ない in the second case.
Yep All OK. As you said, no differences in the meaning. The first two are more polite, the second two are less polite and colloquial.
Then why is が necessary in the first cases? What's this が anyway? Stupid が or Dummy が? No lol, I don't have an answer but..
remember we can sometimes omit particles like が、を and more.
e.g. 歯みがき持ってる？Do you have (a tube of) toothpaste?
We often say like this in conversation, and I think yours is this case. I don't recommend you to omit particles often
Frequency has come back after heavy lumber disc herniation