I think it makes sense that the original deal would allow the king to have long term access to devil assistance but if you don't want to go that direction then I would suggest more then 1 deal. What if the king is constantly having to renegotiate with the devil for more and more help and basically feeling forced into giving increasingly lopsided deals to the devil to maintain power. Ideally the devil's help is also creating some of these problems.

I'd also add it's unlikely the players will ever find out the exact wording of the deal/contract so the specifics don't really matter so I wouldn't put much time/effort into it beyond figuring out motives/goals/relationships between the king and devil(s).


For the keeping the summoning a secret the easiest is to stick with flying devils. Summon them at night and let them fly out of the city unseen to get where ever they need to go. Or have the actual summoning be done by a group of cultists/warlocks, the king works out the details with the devil, and then the devil gets his warlock to do the actual summoning somewhere out of the way.