The hard number for attack speed is then .4? Since 1s/2.5 attacks = .4? This is the number I'm looking for more than 'how many'

The number you are looking for is 2.5. In the bottom left where the stats are displayed, the attack speed is shown in attacks per second.

Quick question about unique item buffs stacking. Does phage's or sheen's unique stack with trinity force? As in, if you built a trinity force, and then also got phage, would the unique on phage also proc in addition to the trinity force unique?

Along similar lines, does sheen stack with lich bane, and does phage stack with frozen mallet?

Also, does lich bane or frozen mallet stack with trinity force?

Generally, items that build into higher tier items will not stack with that item (ex. Sheen doesn't work with Lich Bane, Phage doesn't work with Frozen Mallet). An exception to this would be something like Stinger's unique (cooldown reduction) would still stack with Nashor's Tooth.

However, Lich Bane, Frozen Mallet, and Trinity Force sortof work with each other. From what I've seen you get this (realize that these item pairs are rarely seen, so there is a good chance I'm wrong here):

Lich Bane and Trinity Force

The +damage comes from Lich Bane, not Trinity Force. The rest of Trinity Force's unique still works fine.

Trinity Force and Frozen Mallet

Frozen Mallet will affect the target over the slowing part of Trinity Force's unique, otherwise Trinity Force works as intended.

Does lifesteal work with any abilities?

I'm pretty sure they don't work together.

Gangplank's Parley is the only one off the top of my head that works with lifesteal. The only other is Morgana whose passive acts like lifesteal with her abilities (5% damage dealt with spells gets changed into health).

Is there a difference between magic penetration and a reduction in magic resistance? I generally take them to be the same thing or achieve the same effect but just curious cause of the difference in wording. Would having 15% magic penetration be essentially bypassing 15% of the enemy's magic resistance?

If both were equal values, you'd be correct. However, this is rarely the case.

In the case of Sorc Boots they give a solid 15 magic penetration. That isn't 15%, that's just a straight 15 off of whatever their MR is. Versus something like Void Staff which is 40% magic penetration.

As far as the order in which it is applied (if you had Sorc Boots and Void Staff) I'm still looking for that information.

EDIT: Thanks to TiberiusAudley

I would only assume that anti-MR works in the same way.

Magic Reduction (Abyssal Scepter, Amumu's Passive)
% Based MR Pen (Void Staff)
Flat/Linear MR Pen (Sorc Boots)

Thanks! great info.