Get Stalker/Gustraag 3/Harvester to spawn if I go AFK

The Stalker spawning is completely random, it can happen to almost anyone at any time.
The reason I say 'almost everyone' is because you need to have killed at least one boss since the last time you were Stalked before the stalker can enter one of your games.

The first time I saw the stalker I was playing by myself, and I was relatively new to the game. I doubt I had any last hits on any bosses at that time, though I had beaten some by playing online with others who did the killing for me.

So other than the initial condition of having to beat at least one boss in your entire career, the stalker can spawn at any time, in any game mode, on any player.

No, on each mission you will have to choose a faction to support.

Changing the faction you have supported will subtract from your progress on the original faction until you are back to 0 before adding to the other faction.

So if you complete the mission 5 times for one faction and then 1 time for the other faction, you will be at 4 missions completed for the faction you first did missions for.

And since the reward is not paid out before the node is resolved (100% to one faction), you will not be able to get rewards from both sides even if you play 5 missions for one side and then 10 missions for the other side (I have tried it).

For the event they had where your total missions for a faction counted towards getting special rewards like weapons they said that you would have to have an uneven number of wins, doing 50 for each side would not get you on a winning side even though you then had complete 100 missions.