The keywords are "melee basic attack" and "Warlord power", these are the two criteria that you need to look for in powers in order to tell if they will work with this item or not.
So lets look at each case you're interested in:
No, this bonus is not applied. While the attack made at the end of a charge is usually a melee basic attack, you are granting a charge, not an MBA.
No again. Again, opportunity attacks are typically melee basic attacks, but you aren't granting MBAs, you're granting opportunity attacks.
Probably not. This is one to run by your DM. It's pretty specific that it's "melee basic attacks" not generic basic attacks so I probably wouldn't allow this.
Yes. Your concern (And the concern of the power) is what you grant not what the PC does necessarily. If the PC takes the attack, even if they use another power in place of a MBA, they get to take advantage of the bonus if you can see them.
Again the important thing is to look at the wording of the power, if it tells you that you grant a melee basic attack, you're home free, if it says "basic attack" but does not specify melee, you probably want to consult your DM as it either doesn't work or is pretty situational.