

Well, not directly in the Iliad, no. His death is not mentioned there. Other sources imply he died at the end of the Trojan war, the last couple of days being the topic of the Iliad.
The Odyssey begins roughly 10 years after the fall of Troy, so yes, Achilles would have been dead almost 10 years, give or take a few weeks or months, depending on who you ask.
For completeness, Odysseus himself had already left his home Ithaca 10 years prior to when the Odyssey starts, so roughly at the same time Troy falls.
















Yes, you are absolutely correct regarding the 20 years. I absolutely got myself confused at the end trying to align the timelines.