A: If I won the lottery, I would travel around the world.
Does sentence A mean it’s unlikely that I will win the lottery?
B: I don’t know if he will come to the party tomorrow. If he came to the party tomorrow, that/it would be great.
Does sentence B mean it unlikely that he will come to the party tomorrow?
I keep getting different answers. I would like to clarify what these sentences mean once and for all.
EverestCTSI keep getting different answers.
When it comes to the interpretation of some kinds of sentences in English, wherever there are 10 people to give their opinions on the subject, there will be 15 or 16 opinions about the correct meaning of the sentence.
EverestCTSI would like to clarify what these sentences mean once and for all.
It's not going to happen. Sorry. 
EverestCTSA: If I won the lottery, I would travel around the world.
Does sentence A mean it’s unlikely that I will win the lottery?
No. I'm focusing on the words "sentence" and "mean". For me the sentence does not mean it's unlikely you'll win the lottery. Our general knowledge about the world tells all of us that a lottery is a matter of chance and that it is therefore unlikely for anyone to win a lottery. It is not the sentence that tells us that. You don't need a sentence to tell you that. You know it already.
Rather, the sentence presents the situation of your winning the lottery as an imagined event. Then it goes on to say what you would do next IF that imagined event became a real event.
EverestCTSB: I don’t know if he will come to the party tomorrow. If he came to the party tomorrow, that/it would be great.
Does sentence B mean it unlikely that he will come to the party tomorrow?
No. In this case nothing can be inferred about the unlikeliness of his coming to the party. We only know these facts:
1) The speaker doesn't know if 'he' will come to the party. Therefore, the readers of the sentence don't know either. There is not enough information here either for the speaker or for us to assign any "likelihood score" to the event.
2) There's a possibility that the imagined event of 'his' coming to the party might become a real event, and if that happens, the speaker will be pleased.
CJ
I am completely clear on this now. Thank you, CJ.
Yes, essentially.
Conditionals can be either open or remote.
An open conditional is neutral as to whether the condition is or will be met.
A remote conditional is one where the condition is not fulfilled or is presented as a relatively remote possibility.
The distinction is marked by tense: present tense in open conditionals and past tense in remote conditionals.
If I win the lottery, I will travel around the world....................[open conditional]
If I won the lottery, I would travel around the world............. [remote conditional]
[1] I don’t know [if he will come to the party tomorrow].
[2] If he came to the party tomorrow, that/it would be great.
No: it doesn't mean that at all. [1] is not a conditional construction. The bracketed element is a subordinate interrogative clause (embedded question), in brackets. The meaning is: "I don't know the answer to the question 'Will he come to the party tomorrow?'"
In [2] the past tense marks it as a remote conditional.