DC's obvious problem is that maintaining a majority of 4-5 over a Parliament is on the impossible side of difficult. Short of scrapping the FTPA and holding a snap election in 6 months (which would be punchy), there is one thing he could do to sort this.
The SNP have demanded full fiscal autonomy (so that as well as spending responsibilities, they also have tax-raising ones). The Tories' Strathclyde Commission recommended this anyway - so SNP and Tory policies are aligned. So offer that, and in exchange push through EVEL, ideally with SNP support, if necessary without.
The Tories have an 80 seat majority in England and Wales...