1. [ъ] = [ǝ], [ь] = [ɪ] . Well, that's what I could expect asking the question, but the point is that I would strongly deny the fact [ъ] and [ь] are equal at least to English [ǝ] and [ɪ] (especially [ɪ] in заяц, лягушка etc seemes to be very far from it's English, especially Am. E. counterpart). May be the IPA just didn't want to go too far in trying to reach accuracy? :)
Well, there aren't specific exact symbols for all vowels in all languages of the world -- thank goodness! :) But there are quite a lot of diacritics you can use when you want a more exact transcription. I wouldn't dare try it for Russian myself, though.
2. How about unstressed <и> in syllables immediately before the stress?
I'm not aware of any specific realization of it in this position (apart from cases like с Иваном), but feel free to enlighten me/us.