Haha... all valid points. I don't really have a valid answer. But I'll give it my best!!
1. I absolutely agree. There is a LOT of sloppy code out there in the repository. However, I would not market anything to other people if I wouldn't use it on my own website... and I consider myself to have above-average standards.
2. Thank you for that. This is somewhat of a learning experience to me still. I've only been using WordPress about three months now.
3. I think this observation can best be answered using a multiple response...
a. Yes, there is ALWAYS a possibility for conflict. I wrote the code so that the functions are only executed if the option is selected. So, if there is (god forbid) a conflict, simply unchecking the option will prevent that code from being executed. This way, a user doesn't have to delete the entire plugin, but rather just uncheck the option.
b. Yes, there are PLENTY of other plugins out there with the same capabilities... but would you rather use two different plugins, or have both features in one??
c. I believe there are a lot of WordPress users out there who simply lack the experience of digging through the wordpress repository to find features they may enjoy using. For instance, I never heard of White Label CMS before you mentioned it above.
So yes, the custom login message doesn't really have anything to do with tinymce. However, it is still a nice feature. And for those who didn't know they could add a custom message, this will be a nice treat for them.
Then, if they decide they would like more powerful options, they will be slightly more educated on what to look for. Lastly, they can uncheck that particular option on my settings page, and the code will never be executed, preventing any type of conflict.