It's probably some weird built-in blockchain.info alert. This wouldn't/shouldn't happen in e.g. Bitcoin Core.
I guess blockchain.info has a maximum transaction size. I can't find how many inputs are still on that address, if it's 5000 or so the transaction comes close to occupying a full Bitcoin block.
Doesn't Bitcoin Core allow for manually selecting unspent TX outputs? Try importing your private keys there and give it a shot.
Yes. Settings > Options > Wallet > Enable coin control feature.
When Sending, click Inputs to select them.