I’m here, but I’ve been hoping that somebody else would answer. 8)
You often ask questions about sentences that have no context whatsoever and if my crystal ball isn’t working, it’s almost impossible to answer some of your questions without writing a novel.
The sentences feel wrong to me and raise a lot of additional questions in my mind. For example, the word “would”: Did you mean “would like” or are you using the reported speech form of “will”? If the direct speech was “will”, that raises the question as to why you chose the word “will” instead of one of the more likely future forms. Do you want to make quotes rather than reported speech? Using the word “ask/asked” is preferable when quoting/reporting a question. But if you simply want to repeat a word or part of a question sentence, then “say” would be OK.
That said, here is what my crystal ball came up with:
1- I mean to say, where were you?
If you say “I mean to say”, I would normally understand “I intend to say” and that indicates that you want to say something in the future:
I mean to say something (to him) about the situation soon.
If I wanted to clarify something directly (i.e. in the present), as you said you wanted to do, then possibly like this:
What I mean to say is (that) I want to know where you were.
2- I mean to ask, if you are interested in the deal?
Here again, “I mean to ask” sounds like “I intend to ask” and it doesn’t make much sense to tell someone about this future question when you’re already talking to the person. If you simply want to clarify, I’d write it like this:
What I mean to ask is whether you are interested in the deal (or not).
3 - I meant to say, would you go to the party.
If you first said “Should you go to the party?” and then had to clarify because you said the word “should” instead of “would”, then I would say:
I meant to say, “Would you go to the party”. (i.e., with clear spoken stress on the word “would”.)
4-I meant to say if you would go to the party?
In nearly all circumstances this would be an incorrect sentence. The only possibility as a “correct” sentence that I can think of would be if you simply want to repeat part of a sentence in order to clarify just that piece of the sentence. For example:
I meant to say (that he asked) “if you would go to the party”.
“I meant to ask” is often used in a situation where you had intended to ask something in the past but didn’t:
I meant to ask them if they were coming to the party (but I forgot).