WR841 is not OK. You should check the chart in the following link to make a comparsion. http://www.tp-link.com/en/article/?faqid=465

I'm afraid not. There is a similar thread. http://forum.tp-link.com/showthread.php?78749-Question-concerning-quot-guest-network-quot-when-configured-as-Wifi-Access-Point&highlight=guest+network

I don't think so. You should confirm warranty period with the retailer.

Is it working fine at the very beginning? Did you try to hard reset it and reconfigure it? If it won't work, replace it under warranty.

Did you access those folders wired and wirelessly with the same account? What's the authorization, full-access or read-only? http://www.tp-link.com/en/article/?faqid=680 What's the firmware version?

WAN2 is the backup, isn't it? If so, when WAn1 is activated, WAN2 is deactivated. It is quite normal that you cannot ping it. If you want to ping both, you should not enable link backup.