In MSI on SFDC, you should be able to send the email. You are supposed to have a license to cover the folks in your org who have access. The license really controls the Gmail and Outlook plugins though.
Yes, they can edit the emails, however, you should train them on the process.
For Sales Insight (MSI) each sales rep typically needs a license. The MSI license provide the Marketo tab in Salesforce which to prioritizes leads and give insight into "hot leads", helps you to see activity, interesting moments, see recent web activity, etc... as well as controls the plug-in. It enables Marketing to create pre-defined messages in Marketo (templates) that the sales team can send - like for a mini-campaign or invite to an event, tradeshow, webinar, etc.... And sales reps can always to compose messages on their own too. MSI will also record these interactions as well and sync that to the Marketo activity for the record.
Typically in Salesforce the reps can send messages to their contacts and leads without MSI.