There are people who would appreciate more if you are blunt with what you intend to achieve, so they can make a decision based on it. It could be worse if Takumi found out later and felt like he's being tricked. At least, even if Takumi disagree, they will be fighting this fair and square, which would make it easier rather than having the brother sabotaging his attempts behind the scene.
I think this is the only portion I agree with, to be honest.
I think it depend on how you read into that person.
He doesn't know Takumi is dating his stepsister.
Kakeru also had made him state she is available, and he doesn't like her in that kind of way. So the situation is where Takumi got no good reason to be against Kakeru trying to be her boyfriend.
Even if he wasn't dating her he absolutely does have a good reason to be against it.
Don't forget that moments before he even admitted that he's tried asking her out and has always been rejected curtly. I think anyone with an ounce of sense could put two and two together there and realize "Oh, she's not interested". So, between admitting that and then admitting you're just using someone to try to get closer.
I think it just screams what their true character is actually like. Someone who can't take a hint and/or just can't take no for an answer, and someone tries to use the people/things that the person potentially holds value for to your advantage.
Maybe he's just trying to bait him into admitting his feelings about her or what their situation actually is, or maybe I'm not considering something but I just don't think you can read the situation in a positive way.