And yet, that raises another question: why didn't he look up Geralt much sooner? The moment Geralt arrived in Vizima, or even long before this story starts?
That's difficult because Geralt has a little case of death before this story starts.
That's not a problem at all, considering he's clearly alive when he gets to Vizima and starts investigating Salamandra. Especially when he gets to Old Vizima or enters the swamp would be an excellent time for the Grandmaster to recruit Geralt.
But even before Geralt died, the Grandmaster could have approached him.
Risky, very very risky. For 2 reasons. The first reason is because Geralt has access to Alvin at this point in time as a (mostly) helpless child. If Geralt gets suspicious of the adult Alvin he could kill him as a child and screw his planning over. The other problem could be a an issue of causing some kind of paradox if the older and younger Alvins come into contact with one another. DeAldersburg probably felt it was safest to leave the events up to Chapter 5 as unspoiled as possible to keep the timeline intact. He was nuts but not stupid.
I don't think Salamandra knew exactly who he was as Alvin. There is no indication from the story that they felt he was anything other than an interesting test subject. Alvin for his part as DeAldersburg knew he had nothing to fear as long as the timeline was not mucked with so he stayed the Hell out it and let Geralt save him.
Ironic when you think about it, really.