It's a bit strange, usually we try not to conjugate foreign names in such cases. There are several ways to solve this. Example: You're meeting someone new and asking if it's him. Simple "Sam?" , "Tom?" , "Alice?", will suffice. Or "Czy to ty Sam?", literally " is it you Sam?. Most natural in my opinion.
I believe names aren't conjugated at all, when preceded by być.
Here, there shouldn't be a problem, since the name "Sam" is usually pronounced "Sem".
They certainly are, my quick search confirms that with foreign names it's hugely ending dependant and a bit complicated, should be put into tips probably.
Well, a sentence like "Jesteś Samem?" makes rather little sense because it's like "Are you a Sam?" instead of "Are you Sam?". And the fact that Sam is a foreign name is completely irrelevant, I also won't say "Jestem Markiem".
But then of course "Sam" undergoes declension like every other noun in other contexts, for example "Rozmawiam z Samem" = "I am talking with Sam".
As far as I remember, the male names that don't undergo declension are those that end with -u (in prononciation, so also "Hugh") and short names ending with -o, like Leo. Those ending with -e or -o are problematic because their declension often sounds weird to Polish people. Female names undergo declension only if they end with -a.