I did some Googling... For wealthy commoners, the most reliable way to join the nobility was to purchase a venal office, which would confer noble status. This form of ennoblement was known as
noblesse de robe (nobility of the robe), referring to the robes worn by judicial officials.
- Purchasing an office: French kings frequently sold venal offices to raise revenue. The cost varied depending on the office, with higher-level positions costing more than 50,000 livres.
So, Le Comte didn't necessarily have to be born into nobility. The ability to time travel could definitely allow a person to gain means, although this isn't discussed in the books about Le Comte.
Percy comes from the sub-books, too. His father marries Lord John's mother and becomes his half brother. As both of them are gay, the become fast "friends" ahem, but then Percy does John dirty and their relationship falters. He has been introduced in the last season. He was at the dinner with General Lafayette.
He tries to meet several times with Fergus in the books and when they finally do meet, Fergus discounts everything that he says and doesn't believe him.