It is really cool that the big mek lets you field one deffdread as a troop, but the problem starts when you look at page 90 in core rulebook. Troops which also is a vehicle aren't scoring. So only using kans and dreads wont win you the game. Sorry, but.. to answer your questions.
1. I got some favorites as it comes to arming deffdread and killa kans. For the deffdread I personally use 2 extra cc weapons for giving that extra punch in cc. It can be good for keeping them effective to give them armour plates and riggers, simply to keep them moving forward the enemy;)
Killa kans I use grootzookas on, because, they are awesome, the are cheap and really effective. Their high str makes them effective against most targets, marines, MC, guards, simply everything will die
2. If you make a list with dual mek, dual deffdread and 9 killa kans, we end up arround 800 pts leaving us with 700 pts for the rest. As I see it the army at this point is lacking 2 things. Scoring units and some longe range firepower. For the scoing units I would suggest for this list running some 'uge mobs of boys, I would say 2 groups of 30 would be great. If you keep them within the range of the KFF they will last alot longer, maybe running 2 meks with KFF wouldn't be bad for keeping all the walkers and boys in cover.
The longe range firepower is needed for either destroying enemy vehicles throwing nasty templates or simply stunning them so they can't fire
And whats better for this job than running some lootas, they are simply pure awesome
3. I think a list like this would be able to be competative, maybe not with all 11 walkers, but some number close to that might be able to work.
I hope it gave you some ideas, else send me a message and maybe I can help you come up with some new ideas.