Path of Exile – Is There a Time Limit to Retrieve Items from Pick-only Stash Tabs?

There is a penalty for some levels delta, indeed:

There is a penalty to the chance of currency items (scrolls, orbs, etc.) dropping in areas with a monster level more than two levels lower than your character level. For each additional level that you have compared to the area's monster level+2, the chance of a currency item drop is reduced by 2.5%.

So if you are level 30 in a level 20 area, you will see 20% less currency item drops on average: 2.5*(30-(20+2))=20

A level 30 character in a level 28 area will see no penalty.

Source: http://en.pathofexilewiki.com/wiki/Drop_Rate

The way I understand it: there is a penalty for currency items only, therefore nothing for other items.

The 'nearby' refers to the range of the skill, use the visual animation of the skill as an estimate of the range. This range can be increased through 'increased AoE' passives and support gems (as well as quality% on the enduring cry gem, which gives increased AoE).

"x Endurance Charges granted per one hundred nearby enemies"

With this, for example, if x = 20, then you gain 20 charges for every 100 enemies, or ... in more readable terms, you can 1 charge for every 5 enemies. This is also rounded up, so if you cast it on 5 enemies you get 1 charge, if you cast on 10 you get 2 charges, if you cast it on 6 you get 2 charges, if you cast it on 11 you get 3 charges. You do not have to have 100 enemies near you in order for it to work at all, and of course, when the gem levels up you need less and less enemies to get more charges (since x increases).