Setup alternative DNS entry while DNSimple is down for DDoS?
If you have no access at all to your existing DNS provider, your only option would be to delegate the domain to new nameservers through your registrar. But there would be a propagation delay for that. You might as well do this and you can always switch it back later.
I'd recommend you try out Cloudflare. They have free DNS hosting and will duplicate the records from the existing domain for you. Plus they're very protected from ddos.
No as the registrar is not reachable (as dnssimple is ddos), as it hold your dns entry. You cant change the nameserver