One thing I have tried is right clicking each frame and setting the fitting preferences there.
Have you tried using Object Styles? I don't have time to test, but if data merge overrides the style then it would be easy to script removing the override.
If you apply object styles to all of your place holder image frames, the script to clear any overrides is simple. This is AppleScript:
tell application "Adobe InDesign CC 2014"
clear object style overrides of parent of all graphics of active document
maybe a couple of shortcuts can help you: select the frame and Apple-D on Mac or Ctrl-D on PC InDesign now asks you to select the image you want to place. Locate it on your desktop or in the folder where it's stored and click Open ...
For Fit Content proportionally, the option we use most Often, it's Shift-Alt-Apple-E on Mac or Shift-Alt-Ctrl-E on PC . The same buttons with C instead of E will invoke Fill Frame proportionally.
That only works for placing one at a time, not with Data Merge....