This seams to be a pretty good plugin, thanx for all of your hard work.
I wanted to comment on a few issues I've come across:
are loading into every page on the front side and they are doing so without using the wp_register_script and the wp_enqueue_script.
The same is true for the css files used by this plugin:
Recommended method to load css files via a plugin is with wp_register_style and wp_enqueue_style
By using these functions other plugins such as minify tools will work with your plugin. As it is those files from your plugin are not automatically minified.
Hard to believe that 3 different css files are needed, especially the bs.min.css which is 85k. Most of which if not all is never used by the plugin.
I have also noted that you have created a function: add_bk_option which basically calls the native add_option function. Not really an issue except that you have over 50 options each of which has its own add and get. You can put all of those options into one option as an array then read the array. Just a suggestion.
This Booking Calendar plugin seams to be ok, but you would need to address these issues to be seen as a contender. I see that you have pro versions for sale, you may consider upgrading these issues to truly be considered pro.
Best of luck.