Off topic, but the Rhondda Tunnel group have shown how it could be done.
https://www.rhonddatunnelsociety.co.uk
It could, but the current mess has come about because a £49 bill for a water pump went unpaid. On the western end of the Queensbury Tunnel, they allowed it to be flooded for fishing.
From Shaft Four onwards, the tunnel slopes towards Calderdale, and a council ward where the ward councillors are not prepared to back the project. Shaft Two was filled in due to a lightning strike damaging it. Shaft Three is the one that was actually hit, many years ago, but Shaft Two was easier reach.
No attempt was made to divert the water entering at the Queensbury end, back down the narrow channel built for this purpose. Less than a foot deep and of a similar width. It fed into a natural watercourse on the opposite side.
Any attempt to reopen the tunnel now is going to have all the infill material removed, from below in the tunnel.
A boundary review, under way, might just see the whole tunnel and approaches fall under one constituency. Not the two it does at present.