LinkedIn Tutorial - How to use InMail

How are they see it? And why I can not cancel it?
Here's a weird situation that happened recently. I've sent an InMail for someone and I also connected the person. He didn't replied to my message however he has accepted my invitation. Now I'm bounded to my InMail query and I can not message the person I am connected with. This is not something I call a good user experience.

Hi, I have just started using LinkedIn. I received an email notification to a chat where I had replied on someone's post, however if I try to open the conversation by clicking on join the conversation tab, I can't find it.

Say if i'm the Sales manager for a startup..... Shall i get linkedin premuim on my account or the company account which i manage?