I have an existing single html page layout setup with header and footers already coded and in place.

Is there a way of add the minibb code to this php page directly, such as:

<?php echo mini_bb_header(); ?>
<?php echo mini_bb_main(); ?>
<?php echo mini_bb_footer(); ?>

I'd like to keep the single html template, rather than having to have separate it into header and footer files. This method would make things a lot simplier.

I think I have found out how to roughly do it.

Integrating miniBB in a Wordpress theme

Looks like it'll make upgrading a pain.

Is it possible to somehow like the membership synchronizing make this integration non-code-destructive.

Integrating with Wordpress means synchronizing membership systems. It's not like header and footer. Here we have a detailed guide regarding membership integration.

I also have given reply regarding header and footer integration in this thread:

Is there a way to integrate my header.php file?

If you search by "header" you will find a lot of related topics.