What is the best strategy for a Deadringer and a "Your Eternal Reward" spy on 2Fort?

The main problem you're facing is that you're playing 2Fort and using a setup that doesn't work well on that map. Basically using any different watch or knife would help you get past enemies immensely easier. 2Fort is not a spy friendly map but since you asked....

Without a disguise in the rather claustrophobic map you will be killed quickly so try to get into the water to enter their base. You won't find a lot of safe DR decloak spots near their spawns. The YER works on the idea that the first stab is the hardest to get but gets easier once you have the disguise. Look for a player with his back turned in the doors near the ground level.

Alternatively stand in the corner by the enemies doors on the ground level. When an enemy drops down from above from their spawn you may be able to get a quick stab.

Just to repeat: 2Fort sucks for spies. Going DR and/or YER in 2Fort makes it harder for yourself. Switch up the watches/knives