How can I print subparagraphs?
Seems your preprocess function is the key. To get the value of a field, do the following:

$callout_tab = $node-&gt;get('field_active_callout_tab')-&gt;first()-&gt;getValue();
$variables['callout_options'] = $callout_tab['value'];


Really, please install the Devel submodule Kint and then check first what you get:

ksm($node-&gt;get('field_active_callout_tab')-&gt;value);


Which will most likely get you NULL or an array. So {% if callout_options == 'Single' %} can never work.



And regarding the subparagraph, all I could get was the entity ID:

{{ kint(content.field_paragraphs.0['#paragraph'].get('field_subparagraph').value) }}


but maybe I'm doing something wrong (and you already solved that yourself). But if the entity ID really is the maximum you can get you are better off loading the subparagraph in the preprocess function as well and pass the render array over to Twig like you did pass the bool. In theory this should work :)