You’re not the only one 
https://forum.obsidian.md/t/copy-paste-html-preview-into-gmail/5038
https://forum.obsidian.md/t/button-in-preview-mode-to-copy-html-output-to-new-draft-email/5170
Haven’t heard of this happening. Can you file a bug in Bug reports?
Turn on the Open in default app plugin in Settings->Plugins. This provides an additional command to Show in folder, which will open Finder with the file selected.
Personally, I use Typora. I don’t think you’ll find a consensus on what’s best.
Spell-checker can be turned on in Settings->Editor. Spell check is still a work-in-progress. Check the forum for existing issues.
The short answer is there is no built-in feature for this yet, but it will come eventually either as part of the future Publish feature or plugins. https://forum.obsidian.md/search?q=export%20%23feature-requests%20