This is very weird.

They're asking you for screenshots on completely unrelated exchanges. I would understand if they asked you for the source of your funding, which seems to be the standard AML procedure now. However, there is really no useful information that can be gained from screenshots on Binance and Kraken. You might want to contact them and ask them what's the issue that prompted them to ask for these info. I haven't seen a similar case to yours, and there are many complaints regarding Cex.io here.

Also, fiat withdrawals are indeed a problem because apparently they use a third party service provider for all of their fiat transactions. So you have a chance at getting the withdrawal through, but there is also a huge risk you have to wait for weeks on end for your withdrawal. Thus, it's probably best to only deal with crypto withdrawals on their site. I would suggest you contact cex.io on this forum: https://bitcointalk.org/index.php?action=profile;u=148073, regarding both the reason to why the verification info was requested again, and also regarding the current fiat withdrawal statuses.