Not only, as you say, would it be poison "down south", as it seems it was in 2015 (not helped by Labour joining in the demonizing of Sturgeon and the SNP, but spilt milk and all that), but Scottish (and Welsh and NI) MPs can't vote on "England-only"* legislation after EVEL, so unless that is repealed, there would be key votes where the partner party couldn't have any input.
And one of the sticking points for the SNP in anything more than a confidence and supply arrangement (all that's ever realistically been a prospect) would be Trident and the Trident replacement. I think Salmond might have budged on the existing Trident programme tactically as a bargaining chip during independence negotiations (withdrawal would be a long and fairly complex process anyway). Sturgeon is a lifelong opposer of nukes, and she couldn't do it without losing her own integrity and a vast swath of her voters. Just can't see it happening.
* Scare quotes because the definition of that seems conveniently fluid.
By 2020, Scotland will either be on its way to independence, have already achieved it, or still a no-go area for Labour unless there's a very unlikely collapse of the SNP, or an even more unlikely resurgence of Labour in Scotland (you'd have to see the state of the Scottish Labour leadership close up to understand just how unlikely ...).
Labour needs to focus on its electoral problems outside Scotland. The rest of my attitudes seem very similar to yours.
I've said it before, and I'll say it again, FWIW. Corbyn's less of a problem for Labour than the current PLP taken as a whole. What an incompetent shower of self-absorbed backbiting careerist idiots.