I would like to add on to r13 and 水巷孑蠻's answer.
Zhuxi's central theme here is the problem of annotating classics (經注之禍). Zhuxi was doubtful of Mencius's interpretation of Shangshu (see the bolded parts below). Consider the following somewhat better punctuated text with paragraphs before and after your excerpt.
I also think the 陶弘景 and 唐子西 quotes are only examples to illustrate his disapproval of Mencius's interpretation. They are secondary, in a sense. 陶弘景 allures to 孟子 and 唐子西 allures to himself.
孟子曰：「盡信《書》，則不如無《書》。仁人無敵於天下，以至仁伐不仁，而何其血之流忤也？」曰：「紂一人惡邪？眾人惡邪？眾皆善而紂獨惡，則去紂久矣，不待周也。夫為天下逋逃主，萃淵藪，同之者可遽數邪？紂亡則逋逃者曷歸乎？其欲拒周者人可數邪？血流漂杵，未足多也。」或曰：「前徒倒戈，攻於後，以北。故荀卿曰殺者皆商人，非周人也。然則商人之不拒周，審矣。」曰：「如皆北也，焉用攻？」又曰：「甚哉：世人之好異也！孔子非吾師乎？眾言驩，千徑百道，倖存孔子，吾得以求其是。《虞》、《夏》、《商》、《周》之《書》出於孔子，其誰不知？ 孟子一言，人皆畔之。畔之不已，故今人之取孟子以斷《六經》矣。嗚呼，信孟子而不信經，是猶信他人而疑父母也。」
余隱之曰：《魯語》曰：「俎豆之事，則嘗聞之矣。軍旅之事，未之學也。」孔子之意可見矣。客有問陶弘景注《易》與《本草》孰先，陶曰：「注《易》誤，不至殺人。注《本草》誤，則有不得其死者。」世以為知言。唐子西嘗曰： 「弘景知本草而未知經。注《本草》誤，其禍疾而小。注《六經》誤，其禍遲而大。」前世儒臣引經誤國，其禍至於伏屍百萬，流血千里。《武成》曰「血流漂杵」，武王以此自多之辭。當時倒戈攻後，殺傷固多，非止一處，豈至血流漂杵乎？孟子深慮戰國之君以此藉口，故曰「盡信《書》則不如無《書》」。而謂血流漂杵未足為多，豈示訓之意哉？經注之禍，正此類也。反以孟子為畔經，是亦惑矣。謂《虞》、《夏》、《商》、《周》之《書》出於孔子，人宜取信。《詩》非孔子之刪乎？《雲漢》之詩曰：「周餘黎民，靡有孑遺。」信斯言也，則是周無遺民也。請以此說為證。
Regarding your questions:
自多 means 'to be full of oneself; to boast' (自滿；自誇), where 多 is the verb. To Mencius, 血流漂杵 is a phrase by which King Wu of Chou used to self-boast.
止通只. As explained by the rest.
多 is exactly the same meaning as in the case of 1. As a side note, the online sources I searched did not show 豈示訓之至哉 but 豈示訓之意哉, and I managed to find 《古今圖書集成》 to have used 意 as well (see below or here). Anyhow I would translate (孟子)謂血流漂杵未足為多 as: the fact that Mencius considered '血流漂杵' not as the most self-boastful [meaning there are even worse instances to Mencius]. It is possible Zhuxi was mocking Mencius here, given what we saw in 0.