I’ve found a wordaround that “solves” the problem, but I’m not going to accept this as an answer until I’m not sure it is the only solution (which I suspect it is not).
I can tell WP is not including the Ultimate VC Addons (UVCA from now on) CSS because my page does not contain any UVCA shortcode, or, at least, the initial page content doesn’t. Shortcodes in WP are not recursive. If a shortcode generates output that contains other shortcodes, it is responsibility of the programmer to handle those shortcodes. I actually call
$content = apply_filters('the_content', $content);
but that is not enough to include the required CSS, because the wp-head action has already run by that time. I guess that in the wp-head action, be it in the WP code or in the UVCA code, there must be a snippet like:
foreach ($registered_shortcodes as $shortcode)
if (strpos($page_content, $shortcode) !== FALSE)
output_link_tags(get_dependencies($shortcode));
(beware, this is almost pseudo-code and the identifiers are made up). So, forcing a UVCA shortcode to be present in the page content from the beginning, makes WP (or UVCA) include the required CSS withtout making my code dependent on the UVCA plugin.
I “solved” this by editing my page with my [dw_elenco] shortcode and adding this:
<div style="display: none;">[ultimate_heading main_heading="x"][/ultimate_heading]</div>
That can’t be the best solution around and it reflects a bug for sure. The bug can be in my code (likely), in UVCA code (also likely) or in WP code (unlikely). Moreover the solution is not generic. If, one day, my customer adds some other plugin with other shortcodes that need a custom CSS, this solution will fall short.
However it is the best solution I’ve found so far.
EDIT: I can now accept my answer because, despite not being the only solution, there is no clean solution to this problem. More details in my other, more general question here.