Hiya.
If I was able to write the rules for 5e's summoning, I'd have the "duration" be "Special". A summoned creature would have a short duration without any "purpose", but it could be longer if the "purpose" took longer.
In my RPG (not campaign...actual RPG system I've been writing), spells are given 'duration's' listed as time frames such as "Minutes", "Hours", "Days". There is a maximum time they will be in effect, or until their purpose is fulfilled. For example, a spell that allows the caster and those included to walk on water has a duration of, say, "Minutes". The caster casts the spell and everyone starts to walk across the big lake, which should take about 5 minutes. The PC's get to the other side, step on land, and seconds later the spell ends. Now go back...lets say at the half way mark they are attacked by some nasty lake creature. They spend a few minutes fighting it. Successful, they carry on. They get to the other side, step on land, and seconds later the spell ends.
That's how I'd do summoned creatures. They are summoned for a purpose..."Kill the devil!", "Guard the camp from intruders", "Carry our treasure out of the dungeon", etc. After the task is completed (or some specific duration...such as 10 minutes per level of the caster), the spell ends. In the "Guard the camp" example, a 12th level caster would have the summoned creature for no longer than 2 hours. If, during that time, the camp was attacked by an ogre, the summoned creature would attack it. After the ogre is dead, it would disperse, it's task complete.
I would also simply not allow "buffs" to work on summoned/created creatures. The way I see them, they aren't "real" creatures; they are magically re-created. I wouldn't allow someone to cast a buff spell on a buff spell, so I wouldn't let a buff spell be cast on a summon spell...after all, a summoned creature can be dispelled, right...?
^_^
Paul L. Ming