YouTube – Where to Give Feedback for YouTube Video Editor

Video thumbnails cannot be disabled by the video uploader. Some videos indeed have no thumbnail but that's a glitch. Note that they can upload their own thumbnails if they want to.

For the viewers, YouTube Nativity has no option to disable video thumbnails, and the two main Google Chrome extensions (viz. YouTube Options and Magic Actions for YouTube) to customize YouTube has no option for it either.

No, suggested videos work off the user's Cache or Google account. YouTube looks back on past videos watched by the user and randomly generates suggestions for them. Although, it is possible that one or more of your videos will be suggested more than once.