Your original question actually asked more than one question, so I have been forced to edit the question according to eNotes regulations and focus on just one question. Please remember in future that you are not allowed to ask multiple questions.
When we consider the character of Horatio in this excellent play I believe we have no reason to doubt his position as the loyal friend of Hamlet. When we compare Horatio to Hamlet's other "friends," Rosencrantz and Guildenstern, we see that Horatio is far more trustworthy. He does not allow himself to be manipulated by Claudius and turned against Hamlet, his friend. From the very opening scene of the play, Horatio's decision to tell Hamlet, and not Claudius, about the visitation of the ghost speaks of his loyalty. Throughout the play he has the role of confidante to the young prince, and Hamlet leaves Horatio to survive himself so that he can tell Hamlet's story, in spite of Horatio's wishes to join his beloved friend in death by killing himself:
If thou didst ever hold me in thy heart,
Absent thee from felicity awhile,
And in this harsh world draw thy breath in pain,
To tell my story.
Horatio is therefore entrusted by Hamlet to clear up his "wounded name," and this is a role we see Horatio begin to fulfil with Fortinbras at the very end of the play. At no stage are we given cause to doubt Horatio's dedication to Hamlet.