Sorry I forgot to mention no eshop (see below). What happens is you put emunand into sleep mode with wifi on. You downloaded the update and it will nag you once a day to update. You have a new system with nothing really installed on it correct? If so, in emunand go to system settings and format emunand,Mathis will wipe the update. You will need to run through the initial setup again, nickname, age, region etc.
Yes this means no streetpass. If that is a problem for you then just realize you are going to have that update nag in emunand (I do) and to be careful who you let touch the thing. (99% of the time they will say yes). Also make sure you keep backups of your emunand/mSD card so if it does get updated you can reinstall your backup. In practicality few are able to remember to turn off wifi before shutting the lid and it doesn't take long for Nintendo to push the update to you. It was a bit easier with the o3ds and the wifi switch.
If you get the update nag in sysnand you can format it (without mSD card slotted) or there is a button combo that will take you to recovery mode and you can fix it there. (See tutorials section).
Regarding eshop. You have two options. 1. Get everything you need from the "other" site and install it as cia. (How I have been doing it). 2. There is a new fw spoof method that will get you in eshop. [How-to] Spoof firmware (to access eShop and more) on New 3DS and Old 3DS
Ah well, I think I will just deal with the nag message. I really want to be able to do street pass, I thought it sounded really cool when I heard about it. I also think I will be fine playing local multiplayer for something like Mario Kart, if I ever meet someone with a 3DS - I can live without the online multiplayer.
Here's hoping someone will figure out a flawless CFW, with no worries about accidental updates and perfect online play. haha
Thanks again very much for your help - you are awesome!