You are mainly dealing with being caught in the middle.

I would pursue a legal complaint against "Imax, The feed provider AND the sales rep by name".
You need to clearly explain in your complaint the difference between “presentation” and “content”. It should also be made clear that you are not getting paid due the feed provider's failure to provide content as promised by their sales rep. By doing this you will force all 3 of the others to settle their differences so you get paid.

I'd say you have a couple of issues to think about.

1. With regard to Imax, is really going to depend on whether or not Imax understood clearly that your company would only be passing along the feed to the website. That is to say, you would provide the presentation (style) and not the feed (content). Imax may only have understood the term “website” as a whole.

2. The feed provider did not perform as promised and Imax has their own contract with the feed provider. This should be a clear flag to a judge that there is a difference between what services you performed and what they performed.

3. The feed provider is also ultimately responsible for their sales people's promises. If the rep. intentionally misrepresented or lied about the feed's capability then a judge may decide to share responsibility with the ex-employee. (Dunno. I'm not an expert in this kind of thing, just working off of principles.) The fact that there are signed contracts between the feed provider and Imax means the feed provider put their stamp of approval on the promises of the sales rep. By having the sales rep. listed on the complaint it will force the feed provider to address the issue with their ex-employee. Their may have been fraud on the part of the rep. that needs to be addressed as well.

I believe a judge will look at the ex-employee and determine if any fraud was involved and treat that separately if needed or even if the ex-employee “can” be held responsible. It may depend on state law.

Next the Judge will decide if the feed provider should be held responsible for promises made by their ex-employee. This will usually be yes especially due to the “stamp of approval” effect.

Lastly, the judge will examine the contract between Imax and the feed provider as well as the contract between your company and Imax. The judge will then decide if you fulfilled your part. Since the failure is with the content and not the presentation, it should be clear to the judge that you met your part and since you had no contract with the feed provider, they are not responsible for paying you. This will force the Imax to address the issue with the feed provider. Hopefully, this will put the feed provider on the hook for not fulfilling what they agreed to and put their stamp on. Then Imax has to choose whether or not to pursue reimbursement from the feed provider.

Just my thoughts but I hope it helps.