I think the trouble here is that the only way Labour could win is by aggressively moving towards the centre, which as you describe they are doing. Loosing Scotland to the SNP has completely cut them off at the knees.
What we really need is complete electoral reform and moving to a proportional representational system. Exactly as you describe we no longer have a simple left and right to our politics, Brexit proved that. It will be painful while the main parties shrink and smaller parties grow but I believe the outcome would be a more effective and representative government. Trouble is neither main party could push it though as it would result in them having less power, that's a hard sell.
I did read that each time Labour got in government, it would have occurred irrespective of the parties the Scottish MPs represented. That because the swing within England was so great as to make the Scottish votes an irrelevance (England being about 10x the population of Scotland).