Glycolysis is linear, with multiple entry points. Gluconeogenisis, (and storage as glycogen,) is also linear, but they do feed each other.
There are many pathways we are still working on, but we do have a fairly good handle on energy production and use in the mammalian body.
Carbs, protein and fats do have preferred pathways, as you are aware, but they do share and can (and when needed, will)all be burned for energy. We do get emaciated when not eating for a reason.
With a few exceptions due to metabolic defects due to disease or dna arrangement, most obesity is simply excess calories taken in vs not enough burned.
Our bodies were and are geared to take in all kinds of stuff, and make energy, repair parts, and store the rest as fat. When we take in stuff that costs less in atp to process, we are left with more atp to store.
Not sure I follow your logic entirely...
gluconeogenesis, aside from a few tweaks, is glycolysis in reverse...there is no way to feed into eachother, as they are one in the same, just opposite.
Ironically, however, the key to getting big, either developmentally or pathologically is not burning energy efficiently — it is backing glycolysis up by mucking the up the flux. That is when things (and by that I mean cells) start getting aggresive phenotypes (i.e., cancerous)...fascinating!