Hi Kenalix!
To disable thumbnails in posts, try inserting this code into style.css:
.single-post .attachment-post-thumbnail { display: none !important; }
RSS feeds show unformatted text in case the option called “For each article in a feed, show” in “General -> Reading” admin menu is set to “Summary”. Please try setting the above option to “Full text”. Also, if you input text via the HTML mode, i.e. without the paragraph tags, the feed text will not have paragraph breaks.
Please let me know if this solves the issues.
Sincerely,
Andrew | Satori