Hi Hamish,
Please upgrade both the plugins.
https://buddydev.com/plugins/bp-simple-front-end-post/
https://buddydev.com/plugins/blog-categories-for-groups/

Also after upgrade, Please make sure to visit Settings->BCG Settings and configure it.

Now, coming to your issue:-
1. That is correct, I did not provide any hook to remove that. Please add following code to remove it

function bcg_remove_checkbox() {

	if ( has_action( 'bp_before_group_settings_admin', 'bcg_group_disable_form' ) ) {
		remove_action( 'bp_before_group_settings_admin', 'bcg_group_disable_form' );
	}

	if ( has_action( 'bp_before_group_settings_creation_step', 'bcg_group_disable_form' ) ) {
		add_action( 'bp_before_group_settings_creation_step', 'bcg_group_disable_form' );
	}
}
add_action( 'bp_init', 'bcg_remove_checkbox' );

3. Upgrading front end post plugin and the blog categories for groups will fix the issue.

Please update the code and plugins and let me know if that works for you or not?

Thank you
Brajesh

Hi,
It is available on wp.org now. Please upgrade.

Hi,
It is available now from wp.org. I have just pushed the new version there. Please visit your Plugins->Installed and you will see the notification to update.

Hi Kenneth,
Thank you for your patience. Please upgrade to 1.2.1 and It is fixed. Please do let me know if that works for you or not?

Thank you
Brajesh

Hi Kenneth,
Welcome to BuddyDev.
Which version of BuddyPress & WordPress are you using? Also, are you using latest version of change username plugin?

Thank you
Brajesh