Can anyone tell me if I am right to say that sentence 1 is a simple sentence because "went to the post office" cannot be an independent sentence and sentence 2 is a compound sentence because each part "Bob went to the store" & "Sue went to the post office" are both independent sentences?
1. Bob went to the store and went to the post office.
2. Bob went to the store and Sue went to the post office.
Also, am I right to say that for "unlikely" - "un" and "ly" are the affixes and "like" is the root word?
Both are compound sentences. The subject of the second part of the first sentence is simply implied ("Bob"); it's still compound.
The root is likely, not like. It is not related to the -ly ending found on adverbs (although likely may be used as an adverb, it is also an adjective).