But it was shown/implied. You're just repeating the only argument you have over again. 
Also they were talking about marriage, not their first date. I think they're beyond the step of not knowing how the other feels.
Except it wasn't. Because it's the only argument I need.
No,
they weren't talking about anything, Ymir was. Again, nothing points to them being a couple within the story. It's a headcannon.
There are obviously two characters who like each other in many manga / anime and are not a couple.
The two of them liked each other. In the love letter, Ymir said ' What I'm trying to say... Sincerely... I cannot marry you yet '. One can't say to the other such thing without the other reciprocating.
Which means those two characters aren't a cannon couple.
Yes, you can. It was never reciprocated, ever. Honestly, it came off to me the entire time like they were friends and that Ymir was trying to force herself on Historia. Either way you look at it, there is nothing in the manga that implied that Historia liked Ymir beyond friendship, nor that they were a couple.