Has Jeremy Corbyn’s leadership been a success? It depends on your perspective. Has he successfully reversed Britain’s political and cultural drift to the right? The opinion polls suggest not, but until 8 June we just won’t know.

What about his second, implicit promise on being selected as leader of the Labour Party: to overturn and replace the party’s establishment? After all, the four candidates he has defeated for the leadership share the same background: a stint as a special adviser to a Labour heavyweight before being selected for a safe seat.

If you want to understand how that battle is going and how the party’s power brokers think the election in June will pan out, you need to know one name: Stephanie Peacock.

Peacock, an official at the GMB union, is a Labour candidate in 2017, just as she was in 2015. But last time, she was locally selected to fight in Halesowen and Rowley Regis, a Conservative-held marginal in the West Midlands that was expected to fall easily to Labour. (She lost by 3,082 votes.) Now she is the candidate for Barnsley East, formerly held by Michael Dugher and regarded as rock-solid Labour territory because of his 12,034 majority. Peacock was selected not by party members there but by the nine officers of the party’s 35-strong ruling National Executive Committee (NEC).

This is not, by itself, remarkable. Thanks to the hasty timetable of the snap election, the NEC will select a candidate in every seat where either the incumbent or the defeated Labour candidate from last time does not wish to stand again.

Those nine officers include Corbyn and his deputy, Tom Watson; Cath Speight, who represents Peacock’s union, the GMB; and Glenis Willmott, the leader of the Labour Party in the European Parliament, who has long-standing links to the GMB. Representing the other big three unions are Jim Kennedy from Unite, Keith Birch from Unison and Andy Kerr from the Communication Workers Union. Diana Holland, the party’s treasurer, is also the assistant general secretary of Unite. Ann Black, a fixture on the NEC since 1998, represents the members. This arrangement maintains harmony between the unions – which balance each other out – but it also gives them huge clout as a group. By contrast, Conservative Campaign Headquarters is drawing up shortlists of three names, which are then voted on by local party members. The Lib Dems had already selected close to a full roster of candidates in anticipation of a snap contest in July last year.

These differing approaches reflect both the limits of the parties’ rule books and their respective confidence. The Conservatives aren’t worried about taking a little longer over their selections, as they plan to fight a presidential-style election based around Theresa May’s popularity. The Liberal Democrats selected early because they needed to dig in quickly to overcome their smaller profile in the national press.

In Labour, the fate of candidates such as Peacock tells a bigger story. In 2015, the favoured children of the big unions were put forward in marginals that the party expected to win. This time, the unions have secured Labour strongholds for their candidates, which implies that they are deeply pessimistic about the result.

Labour’s private prognosis is so bleak that, of the 13 seats vacated by sitting MPs who decided not to run again, just seven were considered worth fighting over by the unions. Of these, four were taken by insider candidates sponsored by the GMB: the local councillors Jo Platt in Leigh and Alex Norris in Nottingham North, Ellie Reeves (the sister of Rachel and the wife of John Cryer MP) in Lewisham West, and Peacock (a friend of Tom Watson) in Barnsley East. This suggests that although Unite and its bombastic leader, Len McCluskey, attract the most press attention, it is the GMB which is most influential in Labour’s internal battles.

The GMB was a critical friend of the leadership under Ed Miliband. Under Jeremy Corbyn, it is now merely critical, so the union’s pessimism is not unexpected. However, it is noticeable that its dour outlook is shared by the leader’s allies, who also sought sanctuary in the party’s safest seats. Sam Tarry, the TSSA’s political officer and a key aide on Corbyn’s second leadership campaign, went for Alan Johnson’s seat of Hull West, as did David (a son of John) Prescott, a press officer to Corbyn. Katy Clark, Corbyn's political secretary, tried for Andy Burnham’s seat in Leigh. All three were frustrated in their attempts. Just one of the seven candidates selected so far could be described as a Corbynite.

Some of Corbyn’s allies, however, genuinely believe that the election campaign will end in success. Karie Murphy, Corbyn’s office manager and a close friend of Len McCluskey, was expected to seek a safe seat. Instead, she will be responsible for running much of the day-to-day campaign.

But if she and other optimists are wrong about the coming election, what will Corbyn’s legacy be? That Labour’s 2017 intake will be dominated by party insiders shows his allies’ failure to overcome their internal opponents. More alarmingly for the party’s left, the rules that allowed this to happen were passed when there was still a pro-Corbyn majority on the NEC (it is now hung), suggesting a failure of statecraft.

Nonetheless, Corbyn’s leadership has undoubtedly changed Labour. Few of its new MPs will be Corbynites. However, they will be to the left of the 2015 leadership challengers Yvette Cooper, Andy Burnham and Liz Kendall.

That’s exactly what the trade unions wanted from Corbyn. Yet, with Theresa May unlikely to pursue a pro-union agenda in government, it remains to be seen how high a price they will pay in return.