While I have slowly built a level of trust and respect for my designer/developer, I want to make sure that in the case he jumps ship, I will be able to preserve my website and online presence as usual. Are there any pre-cautionary measures that I can take to ensure that it will be business as usual in the case of a rift between myself and my designer/developer who I have granted complete access to all of my hosting and backend platforms?

The only thing you can really do is make sure you have all the login information for everything you need to run your site. You need to be able to login to the registrar where you purchased your domain and your hosting account. Those are the two main logins, but you should also know the login to FTP into the server.

With the domain you need to make sure you're listed as the person or company that registered the domain and make sure the email listed as the admin contact is one you have access to, but your designer/developer doesn't.

Also make a backup of all the files for your site as well as any database used. You should be making regular backups.

As far as your designer/developer doing something you have to trust that person. He or she needs those logins to work on your site. That person shouldn't use them to be vindictive if your relationship sours, but it's always possible and the truth is there's not much you can do to prevent it from happening. That's where the backups come in. If your designer/developer does something you can put things back to where they were. If you have the login information to all the places I mentioned above you can change those logins and lock your designer/developer from getting back in.

The best way not to have a problem though is maintain a good relationship with your designer/developer. Given that you started this thread, I'm guessing things are already going in the wrong direction so make sure you have those logins and have everything registered in your name. Make backups of everything. And if it comes to it change the passwords on all your accounts.

Obviously you want to maintain a professional relationship with this person, but imagine if he is struck with some obscure tropical disease! Then the rules about protecting yourself are the same.

domain and your hosting account.

This should be mandatory. If you don't have it, ask the designer to email it to you immediately. Be prepared to change the passwords, and maybe you should do this regularly anyway!

Backups come next. We usually email backups to our clients quarterly in a zip file, see if you can get the designer to do the same.

Lastly, I would say, try to scope out another web designer. Not to ruin anything, or to bounce anything around, but just find out who your best mate uses for design. Having a back up designer really can help in a clinch, you just might need to pay through the nose for it.