The verb 'lack' doesn't need a preposition at all. We would usually say 'He lacks patience'. It's a transitive verb, and 'patience' is the direct object. 'He lacks patience' is the opposite of 'He has patience.' We don't say 'She lacks in patience'. This is wrong. It is only the adjective 'lacking' which can be followed by 'in'. It is possible to say, for example 'They are lacking in intelligence'. Here, 'lacking' is adjective meaning 'deficient' - it isn't a verb. 'Sam didn't lack for friends' is correct, but is a rather stilted, outdated construction. I wouldn't bother about it, if I were you.

You're correct about the meaning of the sentences :) As far as the prepositions, I believe "lack in" and "lack for" are phrasal verbs, but I'm not entirely sure about that. Personally, I would never say "lack in"--I would always just say "lack". "Lack for" is more generally accepted than "lack in," but I think it sounds more natural as simply "lack". So, I wouldn't worry about it. Just use "lack" as a transitive verb, and I don't think you'll ever have a problem. I hope this was helpful!