At present, consoles do stack and there is no diminishing returns on them.

So loading up 4 +30 to disruptors will jack up your damage like crazy.

As for armor, the actual resistance % per console should decrease, but it's not technically DR if you understand the math behind how resistances work.

For example if you get a console that gives 30% resistance to everything, then you get another one that povides 30% resist, but is diminished returned down to a cumulative total of 46% resist, it's still roughly the same amount of protection. Then you get another module that diminishes returns down to 55% resist, and it's still the same relative gain in protection the other two offered.

So 30% resist

then the second reduced to only +16% resist

then the third reduced to only +11% resist...

They all provide the same amount of increase.

Most game designers design the diminishing return formula to match up with the actual benefit returned after diminishing returns so that you get roughly the same benefit. I don't know if STO does this or not, but i'd assume they do since almost everyone else does because it's the logical design choice. But then again, as bad as other parts of the game are messed up, who knows, gotta experiment.

Just a disclaimer, those numbers are just rough estimations, I didn't actually calculate them but it's still a relatively small margin of error, you get the idea.