Yes, CSS stylesheets are created by calibre during conversion; but the results are not always optimal. Calibre can see extremely minor variances between material and create extra - and unneccessary - styles which no user could see with the naked eye.
If a book does not look correct, you can edit the EPUB file by hand, by using Sigil. Sigil allows you to edit the text of the book as well as the CSS stylesheet, table of contents and other portions of the book. Removing some of the style variants and correcting the text accordingly may improve the speed the book is displayed as well as correct layout errors. But you do need to correct both: the text so it doesn't call altered styles, the stylesheet to correct/remove styles that are changed/not needed. Once you have a cleaned up CSS stylesheet, you could save it and re-use it for each book you convert, so you would only need to change the text portion of the ebooks you convert after that. (Re-using the CSS stylesheet means you don't have to do as much work for each book that needs to be corrected.)
Sigil is fairly easy to use, but because of the things it can do, it is also easy to make an error. I would suggest only using copies of your converted ebooks.
I'm not aware if calibre can work with all types of text files or if you would have to convert to a specific version. Perhaps that is the reason the spaces between words are lost on conversion. You could try converting one of your text files and see if that fixes the problem. 0x0d, 0x0a (or, CRLF) is the correct EOL.
Lastly, your English is very good. Sometimes my reading and comprehending skills could use improvement though.
