Upon salac activation, what base speed pokemon (beneficial and neutral nature) would a base 90 speed pokemon (beneficial nature) be able to outrun?
With 252 Speed EVs, you will be faster than anything but Jolly 31 IV 252 EV Ninjask and a +Speed Deoxys-S.
is assurance's damage doubled if a foe switches in on me using it and them receiving stealth rock/spikes damage?
Yes.
When restricted by item clause, what are some good alternatives to Leftovers? I'm using Blissey and Hippowdon as my primary Special and Physical walls and both generally enjoy using Leftovers. What are some other options?
Choice Band, Choice Specs, and Choice Scarf.
And whoever asked about weather, here it is, from the Move Descriptions thread:
[NAME]
Hail ***
[SHORT DESC]
Summons Hail for 5 turns.
[LONG DESC]
Summons Hail for 5 turns. Does 1/16 damage to all non-Ice types. Blizzard is 100% accurate (and has a 25% chance to hit through Protect and Detect).
[NAME]
Rain Dance ***
[SHORT DESC]
Summons Rain for 5 turns.
[LONG DESC]
Summons Rain for 5 turns. This boosts the power of Water moves by 50%, and causes Fire moves to do 1/2 damage. Thunder is 100% accurate (and has a 25% chance to hit through Protect and Detect) and Solarbeam has 60 power.
[NAME]
Sandstorm ***
[SHORT DESC]
Summons a Sandstorm for 5 turns.
[LONG DESC]
Summons a Sandstorm for 5 turns. This does 1/16 damage to all Pokemon that aren't part Rock, Ground, or Steel, or have the trait Sand Veil. It also boosts the Special Defense of Rock types by 50%. Solarbeam has 60 power, Thunder has 50% accuracy, and Synthesis, Moonlight, and Morning Sun heal at 25%.
[NAME]
Sunny Day
[SHORT DESC]
Summons Sun for 5 turns.
[LONG DESC]
Summons Sun for 5 turns. This boosts the power of Fire moves by 50%, and causes Water moves to do 1/2 damage. Thunder is 50% accurate and Solarbeam has no recharge turn.