It's not explicitly stated in most comics. But sometimes, character sexuality can be hinted at through subtext or certain interactions. You might need to do a deep dive into the comic series to figure it out for sure.
Frenchie is indeed gay in the comic. This aspect of their character is an important part of the story and adds to the diversity and complexity of the narrative.
Hard to say. It depends on how the comic portrays their interactions and emotions. Sometimes, it might be ambiguous and left up to the reader's interpretation.
Klaus isn't gay in the comic. The creators have not developed his character along those lines. The focus of his portrayal lies elsewhere in the narrative.
Manga is definitely a form of comic. It often features sequential panels and visual storytelling, similar to other comics. However, it may have its own cultural and artistic nuances that set it apart from some common book formats.