The answer to question #1 is "no, you cannot have custom tags inside of attributes of JSTL tags". However, there is a mechanism that permits this type of functionality by using the <jsp:attribute> standard action. See JSP spec for details. NOTE: this is not on the SCWCD exam.
The answer to question #2 is the same as for #1. No tags allow nested tag calls in the text of attributes: not custom tags, not JSTL (which are also custom tags), and not standard tags. "You just can't do it," Frank Zappa (Dinah-Moe-Hum).