I think this is the guide you followed:
They isn't any particular time they update but here's a link for an idea how often nintendo makes an updates:
System Menu - WiiBrew
as for foiling, here's some history:
the first Twilight hack was released then was blocked by the 3.4 update then another Twilight Hack was released and then completely killed with 4.0. Then came bannerbomb which was not stopped in the 4.1 and now even when bannerbomb hasn't been foiled. There's Smash Stack, an exploit with Super Smash Bros. Brawl.
And its not required to update to run newer games you could just extract or download the IOSs that where included in the update so you keep your homebrew and still able to play backups.
There isn't any particular sticky about warnings on the next update but as far as i seen as soon as new update comes out, someone starts a thread about it.
5. Not sure about the emulator from hdd. I know you can play from SD or DVD.
6. Since 3.2 has a very important bug in it for homebrew many apps are developed for it such as Starfall and Preloader but there is now Preloader for 4.0/4.1. So if you have to problems on 4.1 then i dont think there's any need to downgrade.
And you don't need to update when Nintend