Could he be elected PM again?
Not sure how it all works.
No. Here's why:
Let's say Tony Blair wants to be prime minister again. Tony first needs to find a seat in Parliament. He left his seat after he resigned as PM and Labour leader in 2007. Then, he has to trigger the party's leadership contest rules to force yet another contest. So he needs sufficient support from other Labour MPs to challenge Jeremy Corbyn. Then, Blair has to win a contest among party members. The problem here is that other legislators are likely exhausted from the constant fighting, and party members in the country clearly prefer Jeremy Corbyn.
They really dislike the legacy of Tony Blair, especially the disastrous war in Iraq. That's the biggest problem here. Tony Blair is a discredited man who is in it for himself in the eyes of a lot of people. He is like one of the Clintons; we might like some of what he did but he is a relic of a past we also grew sour on. People who might not hate him are still tired of him. So even if he goes back into active politics because he thinks the new guys suck, so what?
Anyway, if he somehow overcomes these obstacles and becomes Labour leader again, the party still needs to fight and win a general election.