I've been out of the website creation loop for a few years now for reasons I'd rather not discuss, but with all of the advances in software and hardware devices, I'm really confused on how I should be creating a website. I last used (don't laugh) CoffeeCups HTML++ and dabbled with Joomla!. I've recently upgraded to HTML Editor v15 but am now reading about RSD. Should I be using RSD instead of HTML Editor or are they designed to be used together?
Older friends and family will probably be viewing the site(s) on either desktop or laptop PCs, while younger family members will most likely be using their smartphones.
Is there any help for the terminally aging?

You will probably need both, at least to begin with. If you had kept your coding skills ship shape, I guess the HTML Editor would have been enough, but as I understand, you haven't been following the latest trends, so therefore RSD should be a godsend for you.
Mind you, there is a learning curve, and while waiting for RSD to be released, you could grab a trial version of RLM(P) to practise with. The two are fairly similar, but whereas RLM only lets you build the 'skeleton' for your site, with RSD you can also fill it with your contents.
The reason why I think HTML Editor will come in handy is, until you have learnt the ins and outs of RSD, you might want to write some custom html or css that you can add to your project after having exported it from RSD (or RLM).

Good luck with whichever way you go! and some of us are always awake to help, just holler!