It's incredibly annoying for a right handed person to place bricks by right mouse click.
That's why I constantly remap a few of my keybinds. My jet key varies between V (in most places) and right-click (mostly in Pecon7's server, because it's easier to go Pyro Jumping when you can have your fingers on all four WASD keys). My plant brick key varies between right-click (when I'm working on something that I have to click to fire the ghost brick) and Numpad / (when I'm using macros or shifting).
Do not combine the knife and the grappling hook together. Who the hell though it would be a good idea to put a grappling hook in knife TDMs anyway. I liked how it use to be when you had just a knife and there was actually a way of falling.
You're missing the point, me and the others have debated this already. It's not a question of is it a good gamemode, it's a question of would people use it. And see my above idea about a vertical grapple knife server taking place on the side face of a huge tower or something. That would be incredibly difficult without the grapple rope + knife.
On another note, I'm going to post in Coding Help about the ways to determine when the jet key is being held and when it's released.