I don't know, how the Barrel family got the title Marquis of Montferrat - till 1708 it belonged to the Gonzaga-Nevers. I don't know, what it precisely meant, possibly an empty title not connected to rights or possession.
But they seem to have lived in Grenoble close to Montferrat.
The last Montferrat ruler ... Ferdinando Carlo Gonzaga, Duke of Mantua and Montferrat (1652 - 1708) ...
https://en.wikipedia.org/wiki/Ferdinando_Carlo_Gonzaga,_Duke_of_Mantua_and_Montferrat
Ferdinando Carlo again chose the French side in the War of the Spanish Succession. In 1701, when the anti-French coalition forces conquered Mantua, he fled to Casale, leaving his consort Anna Isabella Gonzaga behind as regent during his absence. He paid heavily for his choice, when the French were chased back over the Alps in 1706. Already declared a traitor in 1701 by Joseph I, Holy Roman Emperor, he was blamed with felony by the Diet of Regensburg, 30 June 1708, and all his possessions were confiscated.
The House of Savoy obtained the remaining half of Montferrat, having already conquered the first half in the War of the Mantuan Succession in 1631. The Duchy of Mantua became Austrian and ceased its independent existence. Ferdinando Carlo died the same year in Padua.
One half of Montferrat went to Austria, the other half to Savoy. Barral and his title seems to have been a French claim.
In the long centuries before, Montferrat was involved in Eastern claims, starting with members of the Montferrat family as kings or Queen of Jerusalem in 1192.
https://en.wikipedia.org/wiki/King_of_Jerusalem
https://en.wikipedia.org/wiki/William_of_Montferrat,_Count_of_Jaffa_and_Ascalon
https://en.wikipedia.org/wiki/Baldwin_V_of_Jerusalem
https://en.wikipedia.org/wiki/Conrad_of_Montferrat
https://en.wikipedia.org/wiki/Maria_of_Montferrat
In 14th century (since 1306) Montferrat became object to the Greek Palaeologus family, who ruled as emperors in Constantinople since 1259 till 1453.
https://en.wikipedia.org/wiki/Palaiologos
John Palaeologus (heir of Constantinople) married in 2nd marriage Sophia of Montferrat (bride from Montferrat in Italy), who once was promised to Filippo Maria Visconti (strongly involved in the production of Trionfi cards). The marriage was divorced.
https://en.wikipedia.org/wiki/Sophia_of_Montferrat
John in cooperation with pope Eugen arranged the council of Ferrara/Florence 1438/39 (likely very important for the invention of the Trionfi cards (first note of Trionfi cards in 1440, mentioned as a Florentine production)
https://en.wikipedia.org/wiki/John_VIII_Palaiologos
https://en.wikipedia.org/wiki/Sophia_of_Montferrat
Another Montferrat-marriage (around 1440) to an Eastern kingdom involved a Ferrarese delegation, between them a person, who was later involved in a Trionfi-card production in Ferrara 1454 for Borso d'Este.
The right on Montferrat went to the Gonzaga-family in 1536, cause a marriage between Gonzaga and Montferrat.
https://en.wikipedia.org/wiki/Federico_II_Gonzaga,_Duke_of_Mantua
Since Isabella d'Este (if it wasn't already earlier so) as wife of the ruler of Mantova the Gonzaga court was involved in Tarocchi productions (likely in 1512 for Massimiliano Sforza, who reconstituted the Sforza rule in Milan).
There's some rather precise suspicion, that the young Louis Gonzaga, son of the above marriage and later duke of Nevers, was involved in the production of the Tarot de Paris in 1559 together with a young member of the Strozzi (both were then in French service). Family heraldic of Gonzaga and Strozzi appears at the 2 of coins.
https://en.wikipedia.org/wiki/Louis_Gonzaga,_Duke_of_Nevers
The Gonzaga-Nevers family got the rights on Mantova and Montferrat later (1627). One of the female members of the family (later twice the queen of Poland) was responsible for the first known French Tarot game rules (1637, published about 20 years later by Marolles).
And then the Barral family got the right to claim the title Marquis de Montferrat (in 1708 ?)
Well, we have then a family with not precisely defined connections to earlier Constantinople, who married the sister of de la Salette, French artillery general till 1791 and for some time Etteilla's pupil.
Guignard was his cousin, thinking about that, then perhaps the relation to the Marquisat of Montferrat favoured Guignard to become an ambassador to Constantinople in 1768, so it might have been not accidental, that both have had some relation to Constantinople. It's also normal, that both come from the same city, Grenoble. De la Salette also came from Grenoble, that's also not an accidental feature.
From Barral's biography I get, that he also was interested in freemasonry.
"Joseph Marie de Barral, haut dignitaire de la franc-maçonnerie grenobloise, devient en 1770 président à mortier au parlement du Dauphiné et sera très impliqué en 1788 dans la journée des Tuiles et ses suites révolutionnaires. En 1789, il renonce à ses titres et organise avec quelques amis, notamment Aubert-Dubayet qui sera député de l'Assemblée législative en 1791, la Société des amis de la Constitution."
"journée des Tuiles"
https://en.wikipedia.org/wiki/Day_of_the_Tiles
"The Day of the Tiles (French: Journée des Tuiles) is an event that took place in the French town of Grenoble on 7 June 1788. It was among the first of the revolts which preceded the French Revolution, and is credited by a few historians as the start of it."
Grenoble was then a place with 20.000 inhabitants
"la Société des amis de la Constitution" were Jacobins, not so powerful in 1791, but later. Robespierre belonged to them.
https://en.wikipedia.org/wiki/Jacobin
The wedding took place in July 1494, that's the month, when Robespierre was killed. The marriage took place at the 3rd of July, the death of Robespierre at 28th.
Maybe this has not much to say.
Well,we have, that 5 related (somehow "famous") persons are all "from Grenoble". Barral (with Grenoble freemason connections), his new wife and her brother (with Etteilla and freemason connections), his older wife and her cardinal's family and his cousin Guignard.
As all are from Grenoble, it's not very surprising, that they married between each other, but more or less normal behavior.
Giugnard was an outsider of the group, just by destiny. With 15 years he became a soldier, and it's not clear how much time he spend in Grenoble after it. But as a minister he was probably a rather important man inside the family for some time (he was twice a minister).
(second time) "Later, in August 1790, he was also named by King Louis XVI as the Ministre de l'Intérieur."
"... he held office until January 1791." This wasn't rather long.
"Shortly after his resignation he went to Stockholm, where his brother-in-law was the ambassador of the Holy Roman Emperor, Leopold II, to the Swedish court."
"In 1795 he joined King Louis XVI's middle brother, the comte de Provence, at Verona as an émigré minister of the House of Bourbon. After the death of Louis XVI's son, the comte de Provence declared himself King Louis XVIII of France".
"Later, Saint-Priest accompanied Louis XVIII's exiled court to Blankenburg [near Braunscheig, August 1796 - 24 August 1796 - 10 February 1798] and Mittau [then Russia]."
His wife seems to have stayed in Sweden and a lover Aminoff there, as it seems.
They had married 1774 in Constantinople and her father died there in 1793. So her father should have been a very long time in Constantinople.