Link Controller - DNS integration

not sure if i fully understand your question, but you don't have to configure DNS at or for the extra ISPs. you just need one DNS setup (at yourself, one of the three ISPs or even another provider) that points a certain (sub)domain to the link contoller so it can handle it. the link controller will then give back the IP you want it to give back.

rollback depends on your deployment scenario, if you just add a subdomain to your current domain and test with that then rollback is quite easy, just don't use that subdomain anymore. with DNS it is important to configure the TTL (time to live) so entries don't get cached too long. in this case i would put it quite low (i.e. minutes) to make sure that entries time out fast from cache in case you made an error.