It's important to remember the three different paradigms for adjective endings. One for adjectives preceded by a strong determiner, one for adjectives preceded by a weak determiner, and one for adjectives that don't have any determiner. On the first two sentences, you're using the endings that you would use if you had a strong determiner, but there are no determiners. Review my last answer to your question for the corresponding paradigms.
On the last sentence, you're using accusative, which makes sense, since "Experte" is the direct object of a verb, BUT the direct objects of sein (ist, bin, war, ist geworden, sind etc), heißen, and werden take the nominative case because their objects are actually particles of the subject, (ich) instead of a true object. Another way of putting this is that they aren't being acted upon like a normal direct object. You are an expert, you're not doing something to an expert. Make sense?