First Craft website: how to organize different content
I'll try answer each question as best as I can... Feel free to ask for more information if you still have questions. 



First: For your use case, it might indeed by a good idea to put all actual pages in 1 structure, it allows you to better visualise the content structure. You can than pull in the required news, locations, jobs content with corresponding fields.



Second: It depends on how many pages you'll have, if you put it all in one big structure, and you have to many pages in there, you might loose the overview. From what you lined out, I'd put it in 1 structure, and you can still use different entry types if you need different content. 



Third: Singles are ment to be one-off pages, mostly with a specific design, but yes, pages for registering, logging in are excellent candidates for singles. Once you'll use the fields or design on it more often, you might be better off putting it as an extra type in a section. 

Specifically for Jobs, if those are again their own set of entries (I'd put them in a channel) you can use jobs/index.html as template for your overview page (assuming you specified jobs/_entry as path when registering the section).



Fourth: It all depends on how much the content itself will vary between posts, press and media. IF you'll need a different set of fields, you'll be better off creating different entry types. If however they'll all follow the same set of fields, you can work with categories to differentiate between the different pages.