SEO in deeply intertwined with IM.
In the good old days of SEO, one could spam keywords left and right and still rank high. Nowadays, you must invest a significant amount of time (and money) in order to rank at all. That's where the tools and skills you learn come in hand.
And SEO is not limited to Google, either. If there's a search bar, there's SEO.
1. Digital Marketing is just Marketing applied to the digital era, that is, the skills and tools required to sell something to someone online. Simple in concept, damn hard in practice. It encompasses anything from Social Media to SEO and PPC. Each requires a lot of time to even wrap your head around it, let alone master. That's why most people give up early, don't do that.
2. That's a good objective. Think of a website as a storefront to whatever it is you want to offer. Bare in min that building a good website requires a wide range of skills. I'd go with WordPress any day of the week. (see more below)
3. HTML is required if you want to start building your own sites. I don't know much about CSS, Java Script etc. but I believe they're just as important for that.
4. Mastering such wide range of skills will eat a lot of your time. I wouldn't even think about trying other things before reaching a decent level at those skills first.
If I had to start from the very beginning, I'd go with WordPress. There's a high demand for that and I believe its not going to change in the foreseeable future. Pretty much everyone doing business online uses WP.
