Indeed picard was assimilated he is also technically dead
Riker is a weird case as he seems to exist in multiple host bodies
Sisko is definately dead yes (the prophets took him )
Worf merely wishes he were dead instead of living in great dishonour
(he has grown too old to fight )
The whole story of Data's return is detailed in the STO Novel "Needs of the Many". I'll give the short version.
Geordi Laforge and Greg Maddox (from TNG Episode Measure of a Man) have taken B4 in hopes of discovering a way to breakthrough to the Data matrix. Eventually they succeed but discover that both have been permanently written to B4's positronic brain which will eventually overload it. Data decides to sacrifice himself so that B4 can live by deleting himself. B4 decides that Data is needed by the Federation, he stops data and transfers his consciousness to a nearby computer. Data eventually returns to Starfleet to take command of the Enterprise which later disappears.
Again this is from the Official STO Novel "Needs of the Many", which is a good read and gives some insight about the events leading into the game. Enjoy.
"Live Long and Prosper but always carry a fully charged phaser, just in case!". Arrr'ow
Borg share many qualities with vampires zombies and other "low rent" undead
They are corpses animated by evil/a virus/ nanites
they don't have the capacity to reproduce except by infecting a healthy living being
and so on
And its precisely because if you remove our hearts we are dead that Seven is undead
The Borg are Cyborg's, they have technological enhancements, they are not corpses which are been animated by nanites and I have no idea where you are getting this idea from, that is not what a Cyborg is. The Borg race expands itself to perfection by assimilating other races (I don't think using the term infecting healthy beings is erally indicitive of what they are doing). Borg can most certainly reproduce as well, even though they are more likely to use in vitro, we have seen Borg babies afterall in the Best of Both worlds, we also know the borg can do reproducitve acts as Data got 'close' with the Borg Queen. The Borg just prefer assimilation according to 7 of 9.
In regards to Data been still alive (as per the OP's question and not in the philisophical sence). Data's entire collective being was downloaded into B4. To be honest, I see this no more differently than how the Doctors program from Voyager is transfered between ships and to his holoemitter. At the end of the day, Data is a machine and who he is comes effectivly from his memory and programs, since these were all transfered to B4 it's not actually that hard to concieve that Data's personality and being could therefore resurface in B4, although B4 may not be as advanced, it's not hard to imagine that he could be upgraded to better specs, Data made Lal aferall, so he's proven to be able to work with positronics, I'm pretty sure he had all the schematics and knowledge of how to make his emotion chip as well. Even though I think it was pretty weak to kill Data off in Nemesis, I can still see the B4 explanation working without to many issues. (Spock had his memories stored in McCoy and then transfered back again afterall, which is way more a surreal thing to believe than a computer program and memeory been transfered between andriods!).
My views are my own and may not represent those of Perfect World Entertainment