1) Is there a difference between using hal (هَلْ) and a’ (أَ)?
At this level, no in the nominal sentence there is no difference between the two. However, there are some tendencies toward using one of the other depending on the subject. For example if the subject begins with an alif lām it is common to use hal so that there isn’t a long alif sound created (although in some cases this occurs). On the other hand when beginning a sentence with a pronoun that begins with hamzah, such as ‘anta, it is common to use a’.
2) If adjectives are added to the nominal sentence, how can I keep track of the subject and predicate?
Remember that typically the subject is definite and the predicate is indefinite and adjectives copy these aspects from the words they are describing. So make sure you pay attention to the tanwīn and alif lām on the words in the nominal sentence.