Is it disrespectful to write very similar emails to multiple people?
It is not disrespectuful per se to send bulk e-mails. Academics get them all the time: conference announcements, field-specific mailing lists, automated e-mails from submission systems. If it is appropriate for what you have to do, there's nothing wrong with it.

However, I consider disrespectful the way you want to use them. If you just want to know if something is already well-known in the field, start by contacting one researcher. Writing to 100 of them means unnecessarily wasting their time.

Also, it seems like you want to put the introduction of your paper in an e-mail. Don't do that. Keep it short. Your paper should already contain an introduction with motivation and previous research (to the best of your knowledge).

Similarly, for the same reason of not wasting people's time, you want to make sure that your paper is readable to people working in that research area. Make sure that you know the basics of the field and use accepted notation. If your paper looks like it's written by a crank, most recipients will file it directly into the recycle bin.