An alternative DNS root is where someone other than IANA sets up a root zone. At the end of the day, root zone authority is technically not "hard coded". It's a terrible idea to set up an alt root or to use one for these reasons:

1. Security. This is the biggest one. DNSSEC works via setting up Trust Anchors with the root zone and chaining down the tree all the way to the recursive DNS server. DNSSEC doesn't work if anyone in there doesn't have a trust anchor for the root zone. Additionally, if that root zone is untrustworthy, you can effectively have DNS poisoning happen at the root level. Imagine having two google.com's based on which root zone (and therefore walking two separate trees) you ask.
2. It encourages dividing the internet. The two largest Alt zones are Russia's (RNDNS) and China's (.chn). RNDNS exists as a continuity plan in case the rest of the world decides to cut them off of the internet. China's is part of a hare-brained plan to "reinvent the internet under IPv9" (an idiotic plan that sounds even more crazy than Iran's supposed "quantum computer")
3. Pointing to a different root zone can cause a lot of headaches for diagnosing DNS issues when they aren't coming down from the same root zone. It can cause different answers (and a parallel tree).

To answer your second question, they are not good for acting as a way to mitigate DNS failures. No domain servers are going to be asking them in the first place, meaning no one can get there even if it does have the "correct" answer. If all 13 root servers went down simultaneously, the results would be catastrophic. But that's also why they're physically located around the world in many different countries in heavily secure facilities with many High-Availability servers (clone servers that instantly take over if there's a failure, the ultimate "hot" server)

You wouldn't want to have a DNS server ask two root zones anyway. If it can't reach the root zones, then that needs to be addressed. You can't just ask a "less secure" server in case the primary doesn't work. That's just begging for a security breach via cutting off access to the primary root zones so that they "fail over" to the less secure ones.