This part of the book to me felt like Eli was doing everything he could to keep Oskar there, I can't pin myself why exactly but bringing up the walkman when Oskar tried to leave shows this.
My best answer is that Eli was keeping Oskar there till he started asking the questions about Eli that needed to be answered eventually, about the infection, killing people etc, so Eli wants to get it out of the way now because while he may have had it before it's still difficult for him, and when Oskar hit Eli his lack of reaction was because Eli has gotten used to hiding his emotions. The hit may have really upset him or angered him but he didn't want to show this because Eli may see showing emotion as weakness and Eli doesn't want to be weak/can't be for her own survival. Especially because I think Oskar was trying to upset Eli to gain some kind of power or control from the situation and Eli knew this and while wanting this situation to occur still didn't want to end up worse off.
The wrestling after that I have two ideas on, number one that it started out of Elis' retaliation but turned into the two of them venting frustration for each other, but becasue they love each other even while they are fighting they without realising try not to hurt eachother which is why after Elis' initial move he didn't tear Oskar in half.
The second is that Eli thought of it as a way to distract Oskar for a while so that Oskar would stop asking questions, without Eli having to show they are making him un comfortable, plus this allows Oskar again to vent a little of his frustration without doing it through questions intended to emotionally hurt Eli meaning Eli again somewhat has the upperhand.