1. Doing better is rewarded, especially in Warzone. But getting kills is only a part of doing better, you also need to contribute to your team by playing the objective (e.g killing bosses, defending/attacking bases, assist in kills, etc.).
Your buddy might've gotten less kills, but those kills might have had more to do with the onjective. He also could've gotten more assists and a better KDA.
2. The entire point of the REQ system, love it or hate it, is that it is random. Specified packs would kinda ruin the system as it is now. Try to imagine weapon crates in CS:GO where you buy a key for the crate to have a chance at a weapon skin that is worth more than that you paid for, maybe even a $100+ knife. If Gaben decided that you could buy more expensive crates but have a way higher chance at those expensive skins, the entire idea (and market) behind case openings would be done for.
3. All of my yes. Challenges with packs and/or REQ points as a reward would've been very welcome. Only this late into the game with a lot of people already having unlocked eveything there is, it has lost it's use.
4. This idea has been pitched a lot and would be awesome. I don't really know what's holding it back though.