This talk of merging is nonsense; there was never any chance, or desire, for merging. Each party will understandably (if frustratingly) cherish its own identity and specific flavour and won't want that to be subsumed into a vague compromised monolith. Merge was never an option.
What was a possibility, before time rendered it moot, was a specific one-off alliance for these elections under one party banner. Electoral Commission rules are quite strict on this, so a loose coalition alone would not have worked.
That chance has gone now, so the only option left is strategic and coordinated deployment by the parties themselves, or strategic and coordinated voting by like-minded voters. Both are complex and difficult to realise, but the former is much easier than the latter.
I don't think we'll get either, alas; the Remain vote will be split and diffused, and the media narrative will be a win for Leave.
Unless Labour run on a revoke or PV ticket