Keep Niko well back, but not too far back that he loses the drug pusher. Sneak him behind walls where possible. Roman will ring you during this mission, it is OK to answer as Niko whispers down the phone. When you successfully get him to the apartment block, and he goes up the stairs, try and have Niko armed with a shotgun so that he fires as soon as he steps in the door where the drug pushers are holed up, and takes virtually all of the gang out with just a few shots.
No
that all depends on what GTA it is, GTA VC - IV it will mark a person on the map, if you go there the mission will start, older than that i cant help sorry
I believe this is Mission Number 51.
after the first mission with playboy x
depends what mission you are on now
You get nikos phone when you start the first mission
there is no real point to them other than playboys 'photoshoot' mission and a random mission you can do with Jeff
30 look on love meet after out the closet mission
Yes, there is a mission in GTA IV where you rob a bank. It is called Three Leaf Clover. You need a nice suit. You get the mission from Packie
Yes, Gay Tony does appear with Luis in GTA IV. They negotiate a hostage deal with Niko and Packie in the mission "Diamonds Are A Girl's Best Friend". Luis is also seen briefly in the bank heist mission "Three Leaf Clover" and "Museum Piece".
It would take you at least a month or two to beat the game
If you are playing GTA San Andreas or GTA IV games, it might be worth doing a side mission in the game, to see if this gets things started again.