英语简单句句的疑问!原句:Sad to say,this project has turned out to be mo

英语简单句句的疑问!
原句:Sad to say,this project has turned out to be mostly low-level findings about factual errors and spelling and grammar mistakes,combined with lots of head-scratching puzzlement about what in the world those readers really want.请问:1、to be mostly low-level findings是不定式被动语态吗?
2、about factual errors and spelling and grammar mistakes书上解释是宾语的 定语,我想问它可不可以理解成宾补?宾语的定语和宾补有什么区别?3、combined with...作伴随状语,为什也不用combing现在分词短语呢?4、书上说about what in the world those readers really what介词结构作定语修饰puzzlement,其中what引导的从句作about的宾语.我想问,介词可以引导名词代词等短语的介宾短语结构作定语,本句中what引导的是宾语从句吗?介词＋宾语从句的介词结构就不属于从句了吗?这个句子中明明有从句却为何还叫简单句?