Window sills are generally bad in winter from 'drafts' but otherwise are fine, IME. It *is* better to set things back a few few inches from sill at least to prevent 'drafting' of cold air upon things, for sure. Illinois sure does fit this are-of-concern
Ferts are a 'hot-topic', for sure. Improper use of any fert is bad, especially with composted things. 'Fresh' compost (such as manure-based ones) can burn roots *very* easily, but 'seasoned' compost lightly applied is great and gives slow release - but often gives mostly-nitrogenous 'enhancement' if using compost as generally supplied by commercial nurseries/stores selling bags/bulk. Many other 'nutrients' are needed beyond what many commercial composts can provide. Using several sources of ferts variously is best, no doubt. Too much too often of dissolvable ferts is horrid, and leads to many issues, especially when soil is 'thick and retentive' which retains the 'salts' of how the ferts are manufactured. Better to use not-enough than to use too much
, fwiw. And do not add fert 'just because' or because it was assumed
that plant needed it - likely one of the leading causes of plant problems, IME. If plant is not growing (dormant, per se) and using up nutrients, it does not need to have such, if that makes sense.
Fish emulsion(s) are *great* but can stink badly, however a few 'brands' are available that are not as smelly. I forget which ones are indoors-usable (sorry).
Regular use of proper-strength ferts does not give a sudden jolt and then 'starvation' as described, not at all. The nutrients are retained somewhat by the media in soil-mix (if proper mix is used, of course), and taken up as needed
Just wanted to clarify a bit more,