Scotland is lost for at least one more election. Maybe two. Get over it. Get on with realising that winning well in England and Wales (and getting something from Scotland) via real PR and coalition (probably with the SNP) is the only route back to government.
Pick the least worst candidate on offer to lead the party – regardless of where they stand in a pretty-narrow-really policy debate (that’s Yvette btw). Develop a set of new policies with the breadth of appeal you needed but failed to offer at the last election, policies that look after those who look after themselves as well as those who cannot.
Oppose this government. Don’t be holding their coats for them by abstaining when they present harsh, unfair, regressive policies. Be prepared to lose again in 2020. You’ve most likely lost already. Stick with your leader when you do. Be ready to win in 2025.
Or be prepared to spend political eternity as the party for losers led by losers.