We certainly have much more knowledgeable members on finishing around here than I (hopefully they will chime in as well), but here are my thoughts:
If you absolutely do not want yellowing, then you should not be using oil based finishes -- depending upon your application it can be a desirable or undesirable characteristic of oil finishes, but a characteristic it is. How quickly and to what extent an oil finish yellows will depend upon the chosen oil base and the degree of sunlight exposure (which will bleach out the yellow).
So stay with your water-based finishes if you do not want yellowing. From the sounds of your post, you have had good results with spraying water-based finishes, so there is no reason to switch now.
You most definitely do not want to mix a water-based finish with an oil-based finish unless you like finishes with predictably unpredictable outcomes (mix a jar half full of water and oil, shake, and that's what your finish is going to come out like when you spray). You can, however, apply a water-based finish over top a thoroughly dried and cured oil-based finish (but scuff the existing finish if it is gloss).