Ok, so I will try to address all of your questions/points.
Yes, the heter to eat before davening is davka for the sake of the davening. And it's more than just a heter, it's a whole shita. I mention this only because it is kind of pertinent to the whole discussion: preparing to daven.
Correct, I know of no heter to work before davening. For that matter, however, I don't know of anything you're supposed to do before davening besides preparing for davening. Working isn't preparing for davening.
Regarding the zmanei tefilia... ideal time for shachris starts at the netz. I wouldn't daven before then (even after alos which there is a heter for if traveling) and certainly not before alos. My only heter to not daven at netz is if I am preparing to daven and I'm still not ready. Sometimes it takes longer. Obviously it's nice if that happens before sof zman tefila but if I'm not ready, then I'm not ready and I'll forgo the s'char tefila b'zman and daven until chatzos which is still certainly zman shachris. The only question arises when it's already after chatzos and I'm not going to try to make a campaign out of arguing this point. What I will say, however, since it was part of your question, is that come shkia, then it's "ready or not." You can't make a brocha on talis and tefilin after that.