with point 2 you made, i dont think the g.e is a large enough factor to cause people to switch to an entirely different type of gameplay, i cant see there will be a strong correlation of anti-g.e players in deadman mode. Alot of people initially will focus on deadman mode, becaues it's llike day1 of RS again and try get on the leaderboards, but i think it'll die down a bit over time.
In regards to the gp prices, i think they'll fall a little, mainly because people will be spending more on deadman mode initially, but i think the people that switch to deadman will still play 07, like as an alt and play both games simultaneously.
You also need to take into the account of the players that will move over to deadman mode simply to gather gold at the high prices, and swap them for 07 gold, so i think the demand for 07 will remain, but maybe slightly lower at the current rate.
With that being said, as people will focus on deadman mode also, we may find that some of the smaller suppliers (goldfarmers etc) of 07gp may stop and focus on deadman mode, cutting suppply a little and resulting in gp prices being marginally unaffected?
All speculation to be fair, we'll just have to see!