Sylar's getting stronger every day – he can control Matt's body now. Matt's power is all about mind control but Sylar has such varied experience with powers that honestly, I don't think Matt stands a chance.
Samuel is very charismatic – appealing to Tracy's issues and influencing her with Jeremy. Noah and Tracy underestimated Jeremy's guilt, he believes he deserves to die. Being roadhauled is one of the worst ways to die. Jeremy's death changed Tracy's mind about Samuel's proposal. It does explain how Danko got his hands on a compass – it is used to guide people to the carnival.
Sylar is affected by alcohol – problem is that to control Sylar, Matt would have to become an alcoholic. It's completely temporary – now Sylar's in control.
Samuel really doesn't care about innocents – he destroyed the sheriff's station to kill the deputy and his side-kick. It's not the first time, he destroyed an entire mansion of people before.