Hello.
this functionality "ability to auto fill booking form fields from the URL parameters" is not supporting in "Booking Calendar" plugin. Why you was suggest that its have to work ? Here is nowhere was suggested about this. The auto fill booking form fields is working for logged in users, for populating booking form fields data from user profile.

In your case you will be need to make customization if the source code for having auto fill of booking form fields from URL parameters.
The best way to start it in the ../{Booking Calendar Folder}/core/lib/wpdev-booking-class.php file
inside of this code:
jQuery(document).ready( function(){ var bk_af_submit_form = document.getElementById( "booking_form'.$my_boook_type.'" ); var bk_af_count = bk_af_submit_form.elements.length; var bk_af_element; var bk_af_reg; for (var bk_af_i=0; bk_af_i<bk_af_count; bk_af_i++) { bk_af_element = bk_af_submit_form.elements[bk_af_i]; if ( (bk_af_element.type == "text") && (bk_af_element.type !=="button") && (bk_af_element.type !=="hidden") && (bk_af_element.name !== ("date_booking'.$my_boook_type.'" ) ) ) { // Second Name bk_af_reg = /^([A-Za-z0-9_\-\.])*(last|second){1}([_\-\.])?name([A-Za-z0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->last_name).'"; // First Name bk_af_reg = /^name([0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->first_name).'"; bk_af_reg = /^([A-Za-z0-9_\-\.])*(first|my){1}([_\-\.])?name([A-Za-z0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->first_name).'"; // Email bk_af_reg = /^(e)?([_\-\.])?mail([0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->user_email).'"; // Phone bk_af_reg = /^([A-Za-z0-9_\-\.])*(phone|fone){1}([A-Za-z0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->phone_number).'"; // NB Enfants bk_af_reg = /^(e)?([_\-\.])?nb_enfant([0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->nb_enfant).'"; // URL bk_af_reg = /^([A-Za-z0-9_\-\.])*(URL|site|web|WEB){1}([A-Za-z0-9_\-\.])*$/; if(bk_af_reg.test(bk_af_element.name) != false) if (bk_af_element.value == "" ) bk_af_element.value = "'.str_replace("'",'',$curr_user->user_url).'"; } } });

You will be need to rewrite exist code for replacing booking form data from code like this
str_replace("'",'',$curr_user->last_name)
to somthing like this:
str_replace("'",'', esc_attr( $_GET['last_name'] ) )

etc...

Disclaimer. Unfortunately we can not start, right now, some personal customization or custom development, because almost have no free time.
Check more about new features here https://wpbookingcalendar.com/faq/need-new-feature/
Thank you for understanding.
Please note, if you will modify the source code of the Booking Calendar, we will not guaranteed the correct work of plugin and do not support it.

Kind Regards.

Hello.
You can do this in the Booking Calendar Business Small or higher versions of update 8.2
You need to activate this option "Do not show payment form, after submit booking form" at the Booking > Settings > Payment page, if you want to show payment form only after sending payment request by email.
Kind Regards.

Hello.
It’s correct Url if the page, after this url visitor will be redirected to the failed payment page.
Please activate access to that page. Contact support of your hosting if you have troubles with settings access to that url.
Thank you.