www.realtimetrains.co.uk
Won't the Northern line from Tooting Broadway already be going to Kings Cross St Pancras? I've noticed it's usually Morden via Bank and Kennington/Power Station via Charing Cross.
It will; however it's quicker to change to Victoria Line, and very easy at Stockwell as cross-platform
Victoria Line from Stockwell to Kings Cross St Pancras 14 minutes vs 20 minutes on the Northern. If you get a fast connection, you might save 2 or 3 minutes! Each to their own. I'd stay put if I have a seat
As a Broncos supporter living on the WCML - my preferred route is to Euston , then Victoria to Vauxhall then SWR to Earlsfield . The walk to the Cherry Red gate line is about the same as from Tooting Broadway but the Northern Line is quite unpleasant in hot weather. The shortest walk from Tooting is via the back streets - not the main road. Google maps to hand. Either walk can be bused - from Earlsfield being the most straightforward . If you are stopping off at the Corner Pin then Earlsfield is probably closer ! As you say engineering work can mean the Northern is the only option. If you train stops at Watford Junction, then you could try the Southern to Clapham then to Earlsfield. This doesn't work for me as I have a 20-25mins wait for the connection in Watford.