Now, Edward Seymour might desire the throne for himself and separate the personal union between Scotland and England... could he launch a coup of some kind? I doubt he would have the pull for it but the possibility might prevent him from being put onto the regency council. Very interesting.
I think he'd have a decent shot at it. Assuming our posited three-man regency council (Lennox, Carey, and Seymour): Lennox has a pretty firm power base on Scotland, but England would likely be somewhat suspicious of him as a foreigner, and not one they've had a decade or so of warning to get used to the idea of as with King James. Carey's English, and from a notable and influential family, but he was fairly unpopular among the English aristocracy at the time (he was viewed as having been rather an over-the-top suckup), and his OTL power and influence came mainly from the favor of King James (dead ITTL) and King Charles (as a five-year-old, his preferences really won't matter politically for quite a while).
Seymour's family had been very powerful in the earlier part of Elizabeth's reign, and though the family was disgraced and stripped of its titles following Seymour's father's illegal marriage to Catherine Grey, they retained powerful connections. I'm not sure how many of their connections would have survived the explosion, but the name at least would be well-known. Seymour's grandfather (Edward Seymour, 1st Duke of Somerset) was Lord Protector for Edward VI.
Come to think of it, I think the Careys may have been one of the families that were close to the Seymours. Seymour's father's (suspended) title was Earl of Hertford, and the Carey family seat (Hunsdon House) is in Hertfordshire. If so, Seymour may be able to recruit Carey's support against Lennox. This would obviate the need for a coup, since Carey and Seymour could simply outvote Lennox.
Another way for Seymour to shore up his position, were he of a mind to attempt a coup, would be to marry his son to Lady Arabella Stuart. She'd almost married him a couple years previously, but Queen Elizabeth had found out and put a stop to the marriage. OTL Arabella later did marry Seymour's second son William after the eldest had died young. If Arabella married Seymour's heir, it'd merge two of the alternative claims to the English throne, and with Arabella's claim to the Scottish throne (fairly strong geneologically, but very weak politically), it'd provide an avenue for a Seymour coup to maintain personal union.
The advantages to sell a Seymour coup over a Lennox-lead regency for Prince Charles:
- Nationalism and connections: Seymour's English and has established English connections, while Charles and Lennox are Scottish and are relatively new fixtures in the English political landscape.
- Dynastic stability: the main line of the House of Stuart consists of a five-year-old boy and an eight-year-old girl (assuming Princess Elizabeth isn't killed by the plotters ITTL), whereas Seymour is an adult and he has an adult heir and multiple spares (Edward, age 18; William, age 16; Francis, age 14; plus three daughters).
The disadvantages, which sell backing Lennox:
- Political stability in a crisis: the coup would likely lead to war with Scotland if it succeeds, especially if Lennox isn't killed in the opening blow. Not an appealing prospect, especially considerining the disruption and panic following the Westminster Massacre.
- Personal union: Lennox is in a much, much better position than Arabella to keep Scotland in union with England.
I suspect the disadvantages would outweigh the advantages quite a bit in most people's minds, but given the inherent chaos of the situation, I'm very hesitent to say that Seymour couldn't make a respectable attempt at a coup if he were so inclined.