1 - No problem powering up the Hicap with nothing attached to it. Just give it a few minutes after powering it off before connecting it back up to your Supernait.
2 - Better to see if you can trial the DC blocker in your home to hear its effect on the Hicap on your home mains supply. I believe @DiggyGun has had good results with the blocker you suggest.
There is a useful thread here which may help you out