Assalamualaikum,
For any tafsir issues, you can refer to
www.tafsir.com for ibn Kathir's tafsir, or you can also download Maulana Shafi Sahab's Ma'ariful Quran here:
http://www.islamibayanaat.com/EMQ.htm
In short, for the verse about the spoils of war, a kind of dispute arose among the Sahabah regarding the spoils of war because since no clear instructions were available at that time regarding them, they thought that whoever could collect whatever of them would belong to him/her. But then some Sahabas' job was to protect Rasulullah(s.a.w.) so they could not collect the spoils. So in order to solve this issue, Allah first revealed that the spoils belonged to Allah and His Rasul(s.a.w.) so that no-one was wronged. A later Ayat (number 41) of the same Surah Anfal gave clear instructions about how to distribute them.
As for the second Ayat that you quoted from Surah Ahzab, I don't trust myself to explain it properly so refer to the tafsirs I mentioned.
Brother, I advise you not to discuss these issues with non-muslims when you yourself don't know about them. Learn the Deen of Allah first, starting from the Quran, and only then spread its knowledge to those who may otherwise create doubts in your own mind. I know of a person who was initially Hindu and later accepted Islam. And without learning properly about the Deen of Allah he began arguing with non-muslims on different issues. Now, he has reverted to Kufr! If only he realized what his fate will be if he died in this state:
3:106. On the Day when some faces will be (lit up with) white, and some faces will be (in the gloom of) black: To those whose faces will be black, (will be said): "Did you reject Faith after accepting it? Taste then the penalty for rejecting Faith!"
Brother, realize that Deen is too important to jeopardize with arguments, especially without knowledge. First learn Allah's Deen, starting from the Quran, practice it and keep gaining knowledge and practicing upon it, until you feel that you have gained enough knowledge, and faith (through the knowledge) that whatever you face would not affect your Eeman anymore.
May Allah forgive us and protect us and take our lives in the state of Islam.
Wassalam