trees won't hide a clinging xenomorph as well as a ditch would a crouching one, for instance, so mode of attack will be limited to pouncing, or charging.
Although the alien might be spotted if razor was looking directly at it, people (and that probably includes anthropomorhpic cats) very seldom look up without good reason.
Not to my knowledge, but if he's such a tech genius he might be able to make one. And don't say it's only beneficial against an alien, a motion tracking device would be invaluable in a situation like this, and since he has a spare three days to prepare he'd have the time to make it.
Although a generic motion tracking device would be more useful than a thermal-based one, it is not that much more useful. He already has a thermal-based one. However, given that up to three of the fifteen potential contestants are cold-blooded or equivalent (the naga is based off a snake, and kikaidia is an android so it would probably count as cold-blooded), It is quite possible that he modifies his existing scanner to accomodate for these.
Lone aliens, mostly drones, become sentient without a queen.
From what my research has shown up (though I have not seen any of the alien movies), it seems likely to assume that the alien normally has a significant amount of natural intelligence, though in the presence of a queen, said intelligence becomes dormant and the alien becomes subservient to the queen. On the island, there would be no limiting influence of a queen, and thus its own intelligence would come into play. Certainly, it would be at least as intelligent as an octopus (and octopodes can be pretty sneaky buggers when they need to).