Using a Payment Gateway that is not in your integrated Payment widgets

I just want to make sure I understand all of this correctly so I can make the correct development decision. The points I am unsure about are marked with (?)

1. TrustCommerce is not a supported gateway at this time.

2. I can use Thank You page to redirect to a page on my site that could retrieve submitted form data and POST it to another site/page, but according to your Terms, I am not allowed to collect credit card information on a JotForm (?) (nor would I want to store it on JotForm servers, and I don't see a way to collect data for pass-through only(?) ).

3. This means that the only ways I could use JotForm to accomplish collecting payment information for processing through TrustCommerce, would be one of the following:

a) After submit, use Thank You(?) to direct user to another site where they would enter payment info. Possibly I could pass a calculated amount from the JotForm(?).

b) Develop my form in JotForm and download the source. Then I could incorporate whatever payment scheme I want(?). Using the form on my site would store no data to the JotForm server(?), but I could store some data and pass some through to the gateway(?).

Thanks!

2. I can use Thank You page to redirect to a page on my site that could retrieve submitted form data and POST it to another site/page, but according to your Terms, I am not allowed to collect credit card information on a JotForm (?) (nor would I want to store it on JotForm servers, and I don't see a way to collect data for pass-through only(?) ).

You cannot collect credit card information because of the security of credit card information. This information is not stored even on Jorform servers.

a) After submit, use Thank You(?) to direct user to another site where they would enter payment info. Possibly I could pass a calculated amount from the JotForm(?).

Yes, you can redirect users to another website with the calculated amount. But, it has some technical requirements. Please see these articles.
Send Submission Data via Post Method and Thank You Settings
Redirecting Users to a Different Page

b) Develop my form in JotForm and download the source. Then I could incorporate whatever payment scheme I want(?). Using the form on my site would store no data to the JotForm server(?), but I could store some data and pass some through to the gateway(?).

Changes you make after downloading your form's source code may cause the form to run unstable. Your changes will not be known by JotForm. Therefore, your form may not work properly. Your submissions(data) will be sent to JotForm servers even if you download the source code. You can add JotForm to your site without downloading the source code.
Adding a Form to a Web Page