You could give him Ghost Speaker? Then he can do 'at will' what a normal wizard can only do with the time and effort of a Ritual.
Certainly I think that's required - he seems able to see and converse with ghosts without expending effort of any kind. It would probably also explain how he's been able to develop his spellcasting without having any (other) magical senses. As Harry has pointed out previously, anyone could learn to do magic - but without the ability to sense its flows, it's like the blind learning to paint.
He doesn't seem to use any focus items at all. Yet he's clearly capable of pumping power into a rapid ritual - and of conducting that ritual without physical aids, not even a physical circle. Harry isn't skilled enough to simply imagine the circle and let that be enough.
The suggestion to adapt Sponsored Spellcasting, removing the need to have a particular sponsor, isn't a terrible one. It's a bit of an awkward kludge, but it would at least provide a rules basis for Mortimer managing ritual effects "with the speed of evocation". Fast enough to manage some simple effects within combat, in fact.