Writing an article claiming that "it's complex" and "it depends" is the easiest thing in the world; even when these claims are false, few will dare call them out as false.

In my experience good testing actually doesn't require wisdom, or at least there's a lot more value in quick easy principles than there is in wisdom (which is very hard to assess whether it's actually achieving anything).

We have plenty of articles that pontificate and cover everything in shades of grey; actually committing to a stance and giving concrete, actionable advice (that may be wrong for occasional edge cases) is far more valuable.

(To answer your questions: have half an hour's worth of tests, start with purely end-to-end UI tests and convert cases to more unitlike tests as necessary to keep that half hour runtime. Don't ever mock, stub if you must but only after you've tried improving the code so that you don't have to. You already know which parts require more testing and which have been adequately tested, give yourself permission to trust your instincts on that one)