You can choose Logan and still romance Bernard. There is a change in Logan's attitude after you stay with him, although at first is not huge (but yes, he becomes less rude). He has 3 hearts in total btw, with the third one there is a huge change. To know how to get them I suggest you to go to his perfil in the diary, you have the info there.
I don't think that there is another thing like having to choose between two characters to get their heart so don't worry.
About the cocky guard... Yes, you can get a heart with him, but not soon. You get it in an event (main mission) that you will probably finish in day 100+. Until then I suggest you that everytime you see him just to continue submitting and being shameless about being with him. And never dominate him, he doesn't like that at all.
And don't worry about your english, mine is not perfect either.