Okay, cool! I'm not an expert, but I'll try my best to give you good food for thought on your questions!
As far as the spectrochemical series is concerned, pi-backbonding ligands are on the far right side (strong field). This makes sense because the pi-backbonding adds some stability to some of the d-orbitals, further increasing the orbital splitting. So, as long as you don't have any other pi-backbonding ligands, I think you can safety assume that CO is the strongest field ligand. I've seen Ph3P on the high side of the series and so I would say that dppm should also be strong from high structural similarity.
So yes, the pi-backbonding is related to the carbonyl stretching frequency. Since pi-backbonding with CO is a metal d-orbital donating into a pi antibonding orbital of the C-O bond, pi-backbonding is a weakening of the C-O bond. Thus, more pi-backbonding means weaker C-O and hence stretching occurs at lower frequencies.
As for how the other ligands effect the pi-backbonding? This is where I get less confident about my thoughts. I agree though that the effect can be described with orbital theory. Since pi-donation results from metal-ligand pi-bonding, what matters most is likely the ability for other ligands to have pi-interactions with the metal. For your question, you would have to consider bromides, phosphines, and carbonyls.
Another idea I have is a structural trans effect. A strong trans-influencing ligand trans to a carbonyl would increase the M-CO bond length, decreasing orbital overlap, thus weakening pi-donation. So the effect would vary based on the trans ligand in the order bromide < phosphines < carbonyls. I'm not sure if this is an acceptable answer or if considered correct or not by the scientific community.