a/ there are no eps images in your document (but the settings might also affect pdf files loaded in image frames, i don't know... it's possible)
b/ having changed the resolution for the eps export when exporting to pdf, i don't see any difference (but i might have not looked hard enough)
c/ changing the eps resolution should not modify the size of the image, only the number of points defining the image (at the same size). there might be a bug, but i have not heard of such a bug...
d/ i don't which eps resolution you are changing before you change the preview resolution, so i cannot replicate.
e/ you have pdf that are loaded in your image frames and you are using the pdf embedding (experimental feature: the experimental part should not really be a problem for you in this case):
f/ you should share a screenshot showing what the final pdf should look like (it's hard for me to understand what is going wrong... even if i can guess). or tell us exactly which images are wrong (for me the pdf looks exactly like the .sla... like the previous time)
g/ i've never heard from people that images get resized when exporting to pdf from scribus.
h/ since you have "simple" vector drawing in the pdfs you're loading into your document, you might want to import them as vectors and avoid using the pdf experimental embedding as vectors for image frames.
i still think that it's likely that you're doing a wrong manipulation but, of course, i cannot exclude a bug...