Thank you for your feedback.
2) This addon uses a standard converter, markdown-it. I think the issue is not the conversion. Probably the Markdown is simply not *styled* the same as you are used to. Version 1.2.0 adds a feature where you can apply custom styling (CSS) for local markdown files; in the future I plan to add a feature to support standardized and custom styling for all markdown files. Eventually.
1) I am not sure what you mean. I was able to view Markdown with Linux (LF) line endings, and they look just the same. Please open an issue on the project in github and describe the problem you are seeing.

I'm sorry you are unable to use the addon as needed. But I believe the problem is that FF never hands over control of the document to the addon. Check Issue #2 on the github page where there are (links to) configuration steps which have worked on Linux. Perhaps they will work on OSX. Please try them and comment on the issue what steps you took and what happened. Thanks.