When you adjusted the HDRI  so that other added light sources would react more realistically (the candle) it was an outdoor scene. So making a sun to about the same values makes sense. How would you do the same process for an HDRI that was an indoor scene, so that the lights you  add are also physically accurate?

I was experimenting and it seems to me that portals help more with HDRIs that don't have a concentrated bright light source (eg. visible sun ) rather than overcast sky or studio HDRIs with reflected light.

None of the tutorials thus-far have showed us how to get this tri-pane setup and this is something I would like to replicate even after this course.  

Actually I've figured it out, but it might be worth a revision as it sidetracked me pretty bad. I'll leave this here just as feedback.

Some time ago I've seen a video where the artist used an HDRI together with Multiple Importance Sampling for the illumination of a table with a place setting (among this a water glass with caustics) .  When do I need this? Could you please explain the technical background closer?

How can I find out whether an HDRI is normalized or not and how do I have to choose the settings for non-normalized HDRIs?