Please clarify me with these:

1) Poppy - Devastating blow:
"Poppy crushes her opponent, dealing attack damage (+0.6) plus 20/40/60/80/100 and 8% of her target's maximum health as magic damage. The bonus damage dealt cannot exceed 75/150/225/300/375."

Which here is bonus damage?

2) Is the whole damage (physical damage valaue + constant dmg value + 8% dmg value) is dealt as magic damage?

3) So when she crits at Q does the entire damage go as magic damage?

4) Say if she crits at Q with Hydra , does that damage cleaved is proportional?

5) Does Hydra cleave magic damage? eg: Kassadin activating his W and clearing waves

6) Can Nasus / Sion / GP benefit from the minions killed by hydra cleave ..ie . ...nasus Q , Sion E and GP gold from his Q

7) Does pantheon's Q , Eve Q ... cleave from Hydra?

8) Hurricane - Ranged only: what exactly does this mean.... melee can get this item but the passive wont work? or even the AS stat doesn't work ? or the item cant be entirely bought at all?

9) Cass - Deadly Cadence :
"After casting a spell, any subsequent spellcasts will cost 10% less Mana for 5 seconds (effect stacks up to 5 times)."

Does this include Item spells as well?

10) If Malzahar ulti some champ and while the ultimate is still in process if anivia casts her wall in between the champs (across that void channeling)...does that disrupt Malz ulti?

I find #10 funny but unlikely to be true. I don't even think that if you wall malz and force him to be displaced, he'll continue ulting.

Most of these are blatantly spelled out by the skill or item itself reread them and you'll have your answer. Hydra is only procced off "on hit" abilities (and GP's Q which is special case) it does not apply abilities, nor does it activate from abilities other than "on next hit" ones.

#9 item activates are not spell casts.
#10 Malzahar can activate his ult on anyone within range that he has sight on regardless of terrain.

#1 The percent magic damage is the bonus damage I believe.

#2 Everything is dealt as magic damage.

#7 Eve's Q shouldn't activate Hydra cause its a spell. Do you mean her W? (or whichever one gives the ATKSPD bonus)

#8 Melee can buy the item and beneift from the ATKSPD bonus, but don't get the extra 2 shots per attack.

#9 Your question is a bit vague since most items don't cost mana. The only things I can think of are Seraph's and Muramana, are you just asking about those?

1) No idea tbh

2)As far as i know yes the whole this is magic dmg, if you have tri force and you hit with Q and tri force proc the whole thing is magic dmg

3)I think it does magic dmg for the spell only then the crit is physical but it does pop up on your screen at all since her Q cant crit but the AD behind it can

4)The bas AD will only splash since tiamat/hydra is physical only

5)As far as i know it will not splash on magic dmg

6)I know for a fact Sion will gain the stacks if anything dies to him no matter what it was from thus ap sion works, GP gains the gold per kill so if the splash kills then you can gain a lot more gold and Nasus however does not gain this

7)No since neither of them apply on hit

8)The attack speed will work on melee but the passive only works on ranged thus making is useless on melee when getting sword of the divine would be better for melee

9)Item spells don't have a mana cost

10)Anivia wall does not interrupt channeling spells nor will it stop malz ult like that

I have trouble/tough time understanding some of the description (probably reason English is not my native language).

Item activates dont use mana...however please consider this scenario:

Cass uses Item cast then Q, will this Q use 10% less mana ? will Item casts contribute to the stacks?

Does that mean lifesteal totally doesnt work on her Q?

First one, I'm -pretty- sure the flat damage is also affected by the cap.

Second one is so irrelevant that it doesn't really matter, all of Cassi's spells are extremely cheap and on short CDs. Not to mention, which active item would you get on her? Only one I can think of is Hourglass and if you use that I doubt you're relying on the slight mana cost reduction.

Yeah. This spell is actually really interesting in how it interacts with sheen, the bonus damage from sheen will also be turned into magic damage.