there is no way Coinbase would receive it (that is where the screenshot came from) unless it was *somewhere* in the blockchain.
That is not true.
Coinbase will show transactions that have been sent even if they are not yet "*somewhere* in the blockchain". That's what "unconfirmed" means. It means that it is not yet in the blockchain.
My fear is that one of the inputs somehow got corrupted. I do not know nearly enough about how bitcoin works on a technical level but why can the other inputs progress properly even if one fails?
Bitcoin requires that transactions be confirmed in order. Therefore your transaction will not confirm unless/until that input confirms. Vircurex is the only one that can do anything about that. You'll need to get assistance from them. Nobody else can help you.
What are the possible reasons for the input not to confirm?
The input might be from a transaction that can't confirm.
For example, perhaps that transaction is waiting on an input of its own.