Who witnesses a Segregated Witness transaction and what incentive do they have to witness the extended block containing signatures? This video claims Segregated Witness is insecure because no one has an incentive to process (witness) the extended signature block.

https://www.youtube.com/watch?v=VoFb3mcxluY

It claims that as the hashing power increases over 50%, you would be a fool (not a Nash equilibrium) to waste processing power on extended blocks containing the signatures, when the real money is in the transactions containing fees. This means anyone can feed a block with any transaction they want, since no one is checking signatures. (Paraphrased.)

From what I understand, Segregated Witness transaction addresses start with a 3 and are multi-signature addresses. So you have to sign, and someone (who?) also has to sign for the transaction to go through. All of these signatures are placed into an extended block. 


Is the extended block also placed on the main blockchain? 
Who processes the extended block containing signatures?
What incentive do they have to process the extended signature block?
Does the extended block have fees?
Who is the 3rd party signer for segregated witnesses multi-sig addresses?
Since exchanges and merchants won't accept the blocks, miners who mine invalid blocks won't get paid. A miner could take that risk if they wanted to, but it's no risk to the system as a whole. Only the longest chain that consists only of valid blocks is honored by non-miners.

I could imagine a miner trying to mine on top of a new block without first checking the extended block. But they'd almost certainly start checking the extended block and switch if it turned out to be invalid. This takes an acceptable risk of mining an invalid block while ensuring that nobody has any incentive to deliberately mine invalid blocks.