The SNP may well end up having a large number of MPs, and could hold significant negotiating power after the election. That could tip things even further towards devolution, and also it's worth mentioning their demands over relocating the Nuclear arsenal in Scotland.
On balance, I think most people in England would prefer a Lab-Con coalition to Lab+Nationalists. So yes, it is plausible to have such a coalition. On the other hand, this sort of panic-driven response is the sort of thing that got the parties in such as mess prior to the Scottish Referendum. After that one poll that put the Yes vote ahead, and then those "omg pls stay we'll give u anything u want promise xx" responses from the main party leaders that were totally unnecessary. Apparently, the main parties are still misreading things in Scotland. It would have been a no vote even without that pledge, but having made it the parties have backed themselves into a corner. All of their own making. Berks.