What would you suggest are the best attributes to use for Summoning and Banishing Demons ?
A few Attributes have access to the Summon Creature boon. I’d suggest either Energy for a Hellfire theme, or more likely Entropy for a dark magic theme which also gets you access to Animation for particularly powerful demons.
As for banishing, your main choice other than just reflavoring your damaging attacks is Protection, for the Nullify Bane.
Yes, that would work nicely
What would you suggest for creating gates to other dimensions and other universes ?
I am planning to convert “The Laundry RPG” to OL and summoning, banishing and creating (and shutting down) gates are a big part of the game
You can describe any number of existing Banes and Boons using that flavor, but for narrative purposes I’d suggest having it be a generic action roll.
Maybe roll Entropy to create portals, with the GM setting a difficulty based on the target dimension and the circumstances, then shut it down with a Protection roll with difficulty based on the strength of the character who opened it.
Of course, if you’re running a campaign where this is a major part then I’d suggest letting your players decide how to build their characters. Don’t tell them all that they have to invest in Entropy and Protection, let them build their characters how they like (with your help after you’ve explained the setting) and allow them to decide how their own dimensional magic works.
You could also use movement as that is one of the stats that gives you teleport. My thoughts on this is that a portal is a mass mode of teleportation, thus opening or closing a portal could be a high challenge movement roll. Just a thought.