Office 365 Update Channels (Targets, Standard, Deferred Releases)

My take on this is below:

1) We are talking about two different things, one is how Office 365 ProPlus is updated and another is how Office 365 features are introduced into a tenant and early access to these features like in SharePoint Online or OneDrive for Business via Targeted release (previously First Release).

2) Semi-Annual Channel is for the majority of your users typically, in Microsoft's best practices for https://docs.microsoft.com/en-us/DeployOffice/best-practices/best-practices-planning-for-enterprise-managed, 89% get Semi-Annual Channel, 10% get Semi-Annual Channel (Targeted) while 1% get Monthly Channel.  https://docs.microsoft.com/en-gb/DeployOffice/overview-of-update-channels-for-office-365-proplus has more specifics on each channel and the https://docs.microsoft.com/en-gb/DeployOffice/overview-of-the-upcoming-changes-to-office-365-proplus-update-management has intended audiences for each channel. 

3) No, deferred 2 (1701) is end of life next month, deferred 1 (1705) and the latest release 1708 become the Semi-Annual Channel.  Semi-Annual Channel gets feature updates every six months, in January and July with Monthly security updates.

4) Client updates may be version specific and vary like Windows desktop, Mac, mobile.  Backend updates should apply to any supported configuration but may require certain configurations to be enabled.  Just look at Focused Inbox for the desktop and how that required Exchange Online to be enabled, then a tenant change (modern auth) and a client update, all working together to provide this feature.

5) Generally, there is nothing to install, sometimes users will be invited to try out new features or certain features will be promoted in the interface, like the SharePoint mobile app.  

6) No, it's mentioned in your link "They do not apply to Skype for Business and related services."

7) Agreed this is confusing that's brought up here.