using my standing mixer, I creamed the butter and sugar, added the eggs, one by one. Then added some lemon juice, the batter became grainy. I wonder if my lemon bread will be successful ? ( I went ahead and added the dry flour mixture.
TL;DR You should be fine.
Lemon bread, banana bread, etc are quick breads. They are made using the "muffin method". As you said, fat and sugar are creamed together, emulsified with egg, and then incorporated into the dry ingredients.
The fat and sugar are creamed to incorporate air that will help define the final texture. Sometimes the emulsion doesn't form correctly. As you noticed, the lemon caused your batter to separate.
It's ok. You can mix in the dry ingredients. I would expect that the broken emulsion would affect the final texture- but the times that this has happened to me I didn't notice any problems with the final product.
Remember not to over mix the dry and wet ingredients. You don't want a lot of gluten in this type of "bread".