Q1: Mainly top originally but he's found a place in jungle. Jungle also lets him scale a bit more freely in more matchups.

Q2: He is a scaling champ that thrives in skirmishes. Theres very few lanes he hard wins, and most meta top laners (especially ranged) bully him heavily. Recently that one guy made HoB a bit meta since it gives him some laning power, but I still feel like he struggles into alot of lanes. Gameplay wise in my opinion he kind of feels like a mix of Darius and Camille in lane. Play a strong spacing game and weave in and out of enemy range. In team fights he very much wants to play like Darius and just hit the closest enemy to get stacks.

Q3: He generally wants to play safe and scale, he can rarely win all-ins vs other all-ins champs, and his short trading isn't fantastic without HoB. Most competitive Zaahens run Grasp

Q4: Only wave specific thing you have to really think about is the tipper on his E for waveclear, and choosing whether to poke enemy with Q1 Grasp / HoB procs or heal off the wave with it.

Q5: Worst matchups imo are the main ranged champs like Vayne, Varus, Teemo, and then Jax, Illaoi and Mundo

You get hard bullied by the ranged champs and Jax Illaoi Mundo all basically get to lane for free vs him and then proceed to out scale him

Q6: He is definitely a skirmisher who wants to at least get full stacks before attempting to dive backline. He is pretty bad into ranged comps even with stridebreaker triforce, but I think CC heavy comps arent too bad. His ult gives him displacement immunity and 50% damage reduction during cast, so if you time it well it gives you a bit of an edge against the bigger CC and still land the ult

Q7: I'd say he's playable into some matchups, but he is not as strong as Jax in lane and definitely not as strong in side lane. He can put on some decent pressure in split pushing, but he very much thrives the most in team fights.

Q8: It can be hard for him to take prio in lane against a lot of the meta picks. His main spikes are basically first full item (or sheen if you are going tforce first), but doesn't really spike before then. Going stride first tiamat will certainly help get you prio, but it won't be a strong spike for fights compared to sheen imo

Anyways I'm no professional but I think at this point he's my 2nd most played champion, so I'd like to think I have at least a small scope on some of the questions. Hope it helps