The "this is the power of friendship!" line was cheesy as fuck but at least it made sense in context. It isn't like they pulled some deus ex machina for that; the underlying discourse is just "friends support each other and humans are limited when alone".
On harem: I find it extremely implausible too. Two things might happen:
1. OP x Tina. Unlike in the original timeline, Irene is now the queen. MC and Irene can't get together due to duty or crap like that.
2. OP x Irene. Tina becomes the unlucky childhood friend with unrequited love for the MC; never to be married, but never to be together with him, supporting him from the sides.
Either way will be bittersweet. And you know what, I like that.