There must be something else in play. I'm not experiencing the same problem.
Whether using single or double quotes, this:
Code:
<p>A quote where the closing mark is immediately "preceded by a <span>span tag.</span>"</p>
doesn't result in a backward closing quote for me.
No problem with this:
Code:
'circa 1050<small>BC</small>'
on my end either.
There must be something preceding all of that that's causing confusion.
Don't get me wrong; the smartypants algorithm will occasionally result in some backward quotes for me too, but they're fairly rare. And I can't get them to happen with your examples.