LOL you are new to this. ever thing on your site will be fine. 

The current version most definitely will lock out the admin and take the site down if there is any glitch with a plugin update.   

This happened twice to me and made me sad both times but also someone here came to the rescue and saved my donkey (so to speak).
Love,Steve

>>Will the plugins and themes that are currently being sold in the store work "seamlessly"? 

1.6 will consist of two major parts: mobile version and desktop version.
 Mobile version is where almost all changes are done. Since all the "magic" will be happening in a separate folder for mobile version,  updates are not likely to influence your desktop version.

Desktop version was not changed drastically in 1.6 : new plugin, new theme, bug fixes, new folder to deal with mobile version within the core functionality. So it is not likely that updates will break your site as well.

>>Will any failed plugin updates bring a site down and lock out the admin as it is now?

Has anyone tested that?

At this moment this can not be tested since plugin updates will be available only after 1.6 itself is released.

>>I assume that the computer users will see the same as before and the mobile users see the same theme differently or will it must be a mobile theme?  Does the user elect a mobile option or will it be auto-detected.

Computer users will see the same as before.

Mobile users will see mobile version.

System will autodetect users based on their devices.

+1 to FoxTech's replies.

You can never guarantee that update will go 100% smoothly.