OK... so I've taken your information about the people in the campaign (I assume they are all PCs) and did some mental gesticulation.
I'm inclined to make your young Merovingian's blood a red herring. The particulars of how you use him depend greatly on what are the established facts. You mentioned a group, but are unclear as to their involvement (at least as known to the PCs - see below). You mentioned that he's a Bonisagus' son, but didn't mention what is known about the mother or how he came to be known by the father or the covenant.
Have you established any facts around who is doing the framing of Lady Pellegrina. If you're like me, you might start a plot like this with an idea of who's behind it and why, but not setting it down in stone in your mind until actual play (PC's discoveries and knowledge as they develop through the saga) sets them in stone for you. I personally do this because it often is the case that as the PCs go about their protaganist ways, opportunities for 'better' (more serendipitous, more personally meaningful for a PC, more expeditious in the development of other plots) explanations to things that haven't yet been defined in stone may appear that are different than the ones I originally surmised.
The link for Renoir is obvious. His connections to his knights could be a vehicle to feed clues to the PCs as needed. Especially when their actions or investigations 'stall' in the plot. Somehow there should be an issue of justice behind some of the events in order to be able to bring them to the PCs in this role.
The deal with the Oil could go seveal ways. As others have mentioned, the consequence of it having been lost could be nil... but OTH it could be very significant depending on how 'mythic' you like your Mythic Europe.
It seems to me that at the core of this is some sort of 'great discovery'. Presumably this will be about the true nature of successions, the way in which it can or has been manipulated, and the possible discovery of past manipulations.
There could be a contingent of like-minded Tytalus who are interested in setting the whole succession issue up as a contest (for studying) and hid the oil to keep the question of successesion far from clear. The frame of Lady P is to keep the young Lois suspicious and on edge. The more he feels threatened, the more likely that succession could become ugly. It could be that the quasitor's had an understanding on the succession of Lois with Phillip, but that it was discovered by one of these Tytali. Now the Quasitors in question fear acting against the Tytali for fear of their exposing of the interference crime by their own hose. Thus the Tytali feel relatively secure from meddling Quaesitors in their operation.
I'd also think of possibly (if it looks like it could help the plot) somehow involving house Tremere as an interested party. They are extremely interested in orderly succession in general. They are underrepresented in the tribunal, but that says nothing about the possibility of their ability to manipulate these events. My take would be to introduce a Tremere in the tribunal who is a high-up agent of the Exarch in Stonehenge.
Central to all these possibilities should be the question of how things play out from a PC perspective. The PC Quaesitor may have a strange encounter with one of the Tytali where the Tytalus does something obviously suspicious with apparently little regard for how the Quaesitor will react (thinking that his 'blackmail' card will protect him but not knowing that it doesn't apply to the PC). This has the danger that it could reveal too much too quickly depending on the PC. Likely the Tytali will sacrifice the discovered house member in order to maintain the cover-up.
The difficult integration is the Morevingian... more details would be needed to make sure that any idea I come up with didn't already violate any 'immutable' facts known by the PCs. The central question is how do you make him important without doing something over the top. That is, how can Morevingian blood be brought into play without bringing his importance within the Morevingian line to absurd importance. It seems to me that if you wanted to make his Morevingian blood important to the question of the succession, the only way to do it would be to make the character's lineage have a direct impact on succession. The only way to do that would be to make the character's lineage outrageously important... probably too important for my taste. Thus I'd make the bloodline a red herring. The key would be this mysterious group. Perhaps the group is secretly keeping track of all the royal lineages and has a better record of actual lineages than anyone else in Europe. They would be interested in the Morevingian out of hand, but not so much so as to make themselves too overt to the the lad or his acquaintences. They'd be hard to track down. It could be that after all the other machinations are discovered, that parties interested in a 'true and just' succession (a succession based on the 'morally right' interpretation of lineages rather than political situations)
Perhaps the holy oil is required and has mystical significance. Perhaps the oil will kill anyone who is not correctly 'in line'. Perhaps the secret deal with the quasitors dates back to the original 7 generation deal in that the quasitors produced a way to suppress this problem, allowing 'illegitimate' successions if desired. Lois himself could be hiding the oil in order to postpone the succession because the Quasitor's have suspended the deal owing to the discovery by the Tytali.
Ok... that's enough random musings for one night. Hope it's food for thought. Sorry it's not more coherent, but these kinds of questions usually work themselves out for my own sagas by taking an extra long think in the shower. I'm a little more 'seat of the pant's' kind of SG I guess, but I try to navigate an overall plot across the whole saga. The 'seat of my pant's' part is really to keep things as undefined as possible until they are set in stone by events (and become 'the past') in order to give myself the maximum flexibility to adjust course in order to constantly improve the over arching plot in the best, most story oriented way and the most personally meaningful for protagonists.