1 - I did the same and I kinda regretted it coz around 2-3 weeks after, I got the King of Hell summon via free draw. Well for the most part, I regretted it since I didn't have a team in mind at the time, I just want to get the summon purely for bragging rights -- so if you already have a team in mind and you think you can do great with it in activities such as SWB and/or (x)GNW, then go for it.

2 - Personally, I've been using KoH solely to proc 30-hit combo chases -- but now I am into control teams so I rarely use KoH nowadays.

3 - As Jib stated, you only get 4,000 summon coins for duplicates so if you go with your plan on selling all your summon runes, you'll lose out a lot in terms of summon coins -- like me. At the same time, there's also a chance that you may draw Kirin instead of a KoH dupe so...it's really up to you. It's just that, if it was me who's in your situation right now, I'd wait for the SR summon pull and then use all the coins to unlock the other summon.