I have a just discovered a work-around that works for copying and pasting text that is resulting in added line spacings (e.g. making single-spaced lines double-spaced).

If you copy and paste text and it results in becoming double-spaced, if you select the text, then click the icon for "Indent Right" followed by the icon for "Indent Left" (these are the arrows in the format toolbar above the composer area), the selected text will suddenly become single-spaced (and done properly with <br /> tags as well)!

Not sure why this works the way it does, but it has given me a work-around to the problem of copy and paste turning single-spaced text into double-spaced text.

You can of course, edit the html (the second style tag in the header, not the first) with:

<style type="text/css">&amp;amp;amp;amp;amp;lt;![CDATA[
body{font-family:Helvetica,Arial;font-size:13px}
p{margin:0;line-height:1.4;}

Regardless of whether given the option to adjust leading, the *default* leading, as every typographer knows, is optimally between 120% and 145% of the text's point size.
It should never be assumed that line breaks should be double. This is a standard and you are violating it (unless we composing emails as plain text).
Airmail simply needs to define the <p> spacing in the <style> area of its html output, getting the value from either a default or user-defined setting for leading.
The business of email is primarily text and you should consult a typographer if you have any questions about default leading. Please fix. It's maddening to copy large blocks of text that are correctly spaced and have them show up in Airmail as double-spaced.

Does not seem possible to put a single line space in between bulleted text.

You either have to have no space between bullets (which is harder to read) or a double space (which looks stupid).

The +shift+v workaround doesn't solve this issue

Figured out (at least a work around) that may help you. Try this:

CTRL+shift+V

This will paste the text, stripped of all formatting.

indeed