We'd love to! Unfortunately going from just the last email in the thread to being able to reconstruct the entire thread would be exceedingly difficult.
Also, being able to use DKIM / SPF / DMARC / ARC to ensure the authenticity of conversations is very important to us. If you get a permission request from our site, we want you to be 100% confident that what you see when you preview the thread is going to be what you actually wrote, and that the uploader wasn't able to change the text at all. We support making redactions and anonymizing folks, but are strongly against people being able to change the text of what they or others wrote after the fact.
The whole magic of the site is being able to have the experience of looking over other people's shoulders into their inbox, and seeing how they actually talk with one another when they're not on stage. We're all for reducing friction, but we'd rather do it by just making a really slick Add-on so that we don't have to compromise on that core value.
As for the Gmail OAuth situation, I completely agree and have been raising awareness of this on HN since long before this became a big issue the other day. You can see my tickets requesting narrower OAuth scopes here:
https://issuetracker.google.com/issues?q=reporter:alex.krupp...