Questions RE: Remembering user ID, Error Checking out w/ empty cart, Site validation

I have a few questions, if you are able to answer, please consider step by step instructions as I and others who will eventually read this know almost nothing about PHP or mysql, and are just learning our way around ZC.

1. I've had a customer request that my site remember his user ID each time he visits so he only has to put in his password. Is this possible with ZC or available as a mod I haven't found?

2. I've also discovered a user-unfriendly message appears if you try to go to checkout with an empty cart. I get:
   "Whoops! Sorry, but you are not allowed to perform the action requested.
   You are still logged in to your account and may continue shopping. Please choose a destination from a menu."
   I found the above text, and it appears to be a general time_out message that ***may ***be used in more than just this instance. Does anyone know if that's the case? If it's only used in this instance it's easy enough to change - but if it's used in multiple situations, does anyone have any ideas on how to trap the "empty cart" problem before it reaches the time-out and give a more friendly/specific message? OR, could the "checkout" button be disabled or hidden in all locations if the cart is empty?

3. Has anyone put together a site validation test procedure (step by step guide to testing all functions of the site) to ensure all parts of a zen-cart site are working correctly? (at least for the basic installation?) There is so much in ZC that, for a newbie like me, it would be easy to miss testing something.

Thanks in advance for your help.

On #2 - Like everything else in Zen Cart you may change the text to whatever you would like - Please note: that trying to go to the checkout with nothing in your cart is NOT allowed.

#1
Only 1 user wants it? Is he a very important customer you need to keep? You can explain that it's a potential security risk. also his browser should have the ability to do that anyway. It can remember the id he uses on a specific webform.

Thanks - Yes I understand that going to checkout with an empty cart is not allowed (and shouldn't be), and I know I can change the text of the message, my only concern was, would that particular message ever be triggered by any event other than attempting to check out with an empty cart. If so, changing the text to "Checkout is not available because your cart is empty." might not always make much sense. If the empty cart is the only reason this text ever gets used, then it would be no problem. Does anyone know if this is a single use or multi-use message?

Yellow 1912 - ALL my customers are important (brand new store - I only have two so far.) But I think your answer re both the security and the browser capability closes that question for me. Thanks.