1.) His shuriken CD is 6 seconds at all ranks. It falls just under 4 seconds with MAX CDR. If he has max cdr he has no damage unless it's late game and he's full build. Now, most Zeds will build something like a brutalizer which offers 10% CDR. That makes his Q roughly on a 5 and 1/2 second CD. At rank 2, your Q CD matches his and that's not even considering any CDR you have built.
He will not have most of his damage up every 4 seconds because most of his damage relies on his Shadow which has an 18 second cool down at first rank and 14 seconds at max rank. Even with max cdr, that's still around 9 seconds on his shadow. If he hits you with an E, the CD of his shadow is reduced by two seconds.~7 seconds.
I don't know if you're bad at math or just making up numbers to exaggerate your case. Either way, you're still wrong. Plus on top of the low cooldown for your Q, when you hit max Ferocity your CD is essentially reset because it is counted as a separate ability.
2.) If he ults, build max Ferocity(if you know how to play Rengar you should always be hovering around 3-4 anyway) and hit him with an Empowered Bola, rooting him in place. Walk away. Congrats, you just successfully disengaged. Just as the snare is about to end, you can even hit him with a regular bola to apply a slow further increasing the already strong likelihood that you have made a successful disengage. Alternatively, you could try, as he is ulting you, to run into a bush so that you can jump onto a minion and create a sizeable distance between you two. Follow that up with a Bola.
3.) When you go to engage on him, make sure you have 3-4 ferocity. When he shadows away, hit him with an empowered bola and catch back up. Then burst him down. He just wasted a huge part of his damage trying to escape and caught him with a root. He can't go back to his shadow a second time and unless he has ult he has no way to escape. Ideally, you'd be doing all this while you also pressed R so that you can get another round of burst + a gap clear.
4.) Dodge. If you can't dodge a skill shot then Zed is the least of your worries.
5.) This should be the other way around, tbh. If he ults you and then you ult... guess what? You're invisible. He can't fight you. As a result his death mark does little damage and he wasted everything trying to instagib you while you have the perfect leap from invisibility to engage on him with, and crazy sustain between your W heal and the atkspd increase of your q to proc more life steal.
I can't say for certain, but I'm also willing to bet that Zed would wait for you to blow your CDs and then engage on you when you're helpless. You need to think about what's going on as well. You can't just hope to win because you have stronger abilities or something. The strongest abilities in the game are worthless if you aren't intelligent enough to use them.
EDIT: Oh, and this I forgot about... but his Q does significantly reduced damage as it passes through multiple targets ( a la your minions) so it's really your fault if you're positioned so poorly that he can get a clean shot off every time.
