The only way to properly compensate for changes in load is to actually measure these changes. Reacting on the effects will always leave you a step behind. Differentiate the measured load and you know what's happening; you could switch to different look-up tables to compensate for specific load variations.
Secondly, your A/D might be influenced by your switcher, unless you have a suitble low-pass filter on the input (which is not mentioned in your posting) of the A/D. I have seen similar problems on generator voltage regulators that need to operate on a "dirty" bus, e.g. when large frequency controllers are used that create harmonics in the 7th and above order. The result generally is either unstable regulation or large offsets in the regulated output.