How can I import one custom block into another?

You cannot embed blocks as react components this way, and you should not try to use other blocks as user interface components by setting innerblocks with a locked template.

Instead, use 2 filters to modify the core/embed block:

• Use editor.BlockListBlock on core/embed to modify the edit component, wrapping it in your own component that has extra user interface react components
• Use the blocks.registerBlockType filter to modify the core/embed registration to include your additional attributes
• Use PHP to adjust how the core/embed block is rendered using filters

This way you can use the standard core embed block and no longer need to register new blocks.

For example this adds a button to the toolbar of every block:

import { BlockControls } from '@wordpress/block-editor';
import {
    ToolbarButton,
    ToolbarGroup,
} from '@wordpress/components';
import { createHigherOrderComponent } from '@wordpress/compose';
import { addFilter } from '@wordpress/hooks';

addFilter(
    'editor.BlockListBlock',
    'verism/toolbar-button',
    AddToolbarButton,
    10
);
const AddToolbarButton = createHigherOrderComponent( BlockEdit => {
    return ( props ) => {
        if ( props.name != 'core/embed' ) {
            return <BlockEdit { ...props } />;
        }
        return (
            <Fragment>
                <BlockControls>
                    <ToolbarGroup>
                        <ToolbarButton
                            className="verisms-really-nice-button"
                            label="Verisms really nice button"
                            onClick={ () => {
                                console.log( 'button pressed' );
                            } }
                        />
                    </ToolbarGroup>
                </BlockControls>
                <BlockEdit { ...props } />
            </Fragment>
        );
    };
}, 'AddToolbarButton' );

https://developer.wordpress.org/block-editor/reference-guides/filters/block-filters/#editor-blockedit

You could insert sidebar panels, textfields, toolbar buttons etc.

You could also filter the attributes in PHP via the block_type_metadata filter:

https://developer.wordpress.org/block-editor/reference-guides/filters/block-filters/#block_type_metadata