What is the best way to do an instant offline google-like search on all text files on android?

It must show a single file multiple times if the search term appears in the file multiple times.

Ideally it would show results tightly, as many instances as possible on a small display, just like Visual Studio does on desktops.

maybe something here.
i found aGrep which seems to at least partly answer your question.

Is there an android emulation that runs under linux or windows so you can test-drive all these apps before taking risks on your actual smartphone?

yes, several solutions exist for that, but they're pretty hard to set up (at least on linux. windows, couldn't say).

you can always do a factory reset on your actual phone.

i take it your phone isn't even rooted?
you might want to search the actual google play store for file search utilities, but i have a nagging suspicion that 99% of all android users wouldn't even know what to do with sth like that, which means that there's little incentive for app developers.
android DOES have a terminal running bash, maybe you can just use 'find'.