How much of our resources do we get back if we destroy a room

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.

They're slotted as other mods.

Your frame's mod slots will have four slots with an '=' in the upper right corner. These indicate that they are Polarized so that a mod slotted there will only require half as many ranks to slot as its cost would indicate, and the '=' type marking is reserved for frame powers.

If you're only just starting out, you won't be able to slot all four of them yet (you will have all four in your inventory; they're gained at the same time you acquire the frame that uses them), but leveling up your Frame increases the number of slot ranks available.