I'm changing the way several investiture ceremonies work; that will take care of this:
The DDC for the recognition ceremony, for example, will be set to 5 + province level + recipient's new holding level (for holdings). +5 DDC if you have no holding of the same type in the province.
That brings it in line with the DDC calculation used for Rule actions (quire neat). It will make the trading of multiple holding levels in high-level provinces much more costly, without inventing a completely new balancing mechanism.
Hmm, can't decide if I want to increase the DDC by another +5 for the rare occurrence of uncontrolled provinces/holdings.
Btw: The investiture spell will no longer be required for investiture. Rather, it will make investitures more likely to succeed. But I expect that priests would usually be present anyway (as a Free or Court action?) for cultural reasons...Haelynic priests for lands and such, Sarimite priests for guilds or trade, perhaps even one of Ruornil if godly sorcerers are involved.