WOW! These are a lot of questions that can be answered by yourself. Everyone would have different configurations depending the needs of their store as far as the settings in Payflow Link Manager go. That said, I will try to answer most of your questions as best as I can...
1. If your shop doesnt have SSL and you are concerned with clients inputting their CC info, you can A) configure the text that is on top of the form where clients input their CC info (see the screenshot attached in the download above). Its the text that says "CC info is not stored on our system, blah blah blah". I added it so anyone can add more information they might need, such as in this scenario. OR you can B) get an SSL certificate on your website. $69 a year at godaddy.
2. Payflow Link redirects or "links" to another secured page when a client is checking out. So a "test" option is not needed in this extension. Therefore, all "test & live" settings are handled at the Payflow Manager level. If you select "test", no transactions will go through whatsoever because its in "test". Remember to read the instructions included in the download for testing purposes.
For your Display options:
It depends how your page looks. For example, if you have a black background, choose a black background, load your shop logo and have it centered at the top, and customize the pages text to whatever you need it to be. I would advise having "Continue" as the text for the last button so it forces the client to "Continue" back to your website where they get redirected to the opencart success page.
Billing/Shipping Information Section:
This really depends on your needs. For me, I have ALL the shipping attributes UNCHECKED because I do not ship anything to the client. They provide me with their family members address in Mexico within opencart and I sent it to them directly. So everyones shop will be a little bit different. Thats the magic of settings, you can customize it and test it however you like. If its a simple shop, then I would of course require the client to give you ALL of there billing and shipping information based on what you need to operate the business efficiently.
Payment Confirmation Section:
Header Text shows right below your logo. I put the following in mine "Thank you for your order. Please review your order and print this page for your records." Footer Text I put the following "You will recieve a confirmation email to the address you provided. If you wish to recieve confirmation by phone, please contact us at 1-800-XXX-XXXX".
FOR THE RETURN URL & SILENT POST
ARE IN THE INSTRUCTIONS
IN THE DOWNLOAD**
Email confirmation section:
I put no because OC already sends an email automatically, If you select yes, then they will get TWO emails. One from your OC shopping cart and one from payflow link payment confirmation.
AVS (select FULL) CSC (select FULL) and leave all "accepted URL's BLANK".
REMEMBER, the download above is obsolete as it has NOT BEEN UPDATED for the fixes people were needing. You can download the NEW AND UPDATED version here: http://www.opencart.com/index.php?route ... on_id=6033