The luck blade (DMG, p. 179) says:

you [...] cast the wish spell from it.

The wish spell may be cast from the blade, but you are still the one casting it. Therefore, you would still suffer the negative effects of wishing for anything besides copying a spell:

The stress of casting this spell to produce any effect other than duplicating another spell weakens you. [...]

The same applies to any other magic item that says "you cast" the wish spell from it.

Gauging from the lists on all the staves with which I have read the pattern seems to be one charge per level of the spell slot used to cast the spell. If you look at the Fireball 7th level(7c) version and the Wall of Fire(which is a level 4 spell it takes 4c) stands to reason that Finger of Death would take as many charges as the level in which you would want it cast.

To your point there is not much of a power discrepancy between a lvl 7 Fireball and a Finger of Death since one is single target and the other is AoE so potentially the Fireball can cause much more damage overall.

There seems to be a discrepancy on the Conjure Elemental ability though, it states 7c to use but this could be an error and is intended for the casting of at a 7th level slot.

There is no hard RAW details on this but it is extrapolated from the staves.

Wands play by different rules obviously. They cost one charge for the base cast and in the case of some like Magic Missiles (DMG 211) as you mentioned it costs an additional charge to add the equivalent of a spell slot to the casting.

One other thing to consider is the length of time to create a legendary item is quite long. If you go by the guidelines from the DMG it would take a single caster greater than 54 years to create a Staff of the Magi. DMG p129

500,000 / 25g per day = 54.795 years. This is prior to the optional rules in Xanathar's Guide to Everything on crafting.