1) Bruce, it does not matter who the beneficiary is, but it must be an individual, not an entity like an estate.
2) A beneficiary of age can do a QCD whether there is an RMD due or not, but the tax benefit is greater if it offsets RMD taxable income. If there is no RMD due to the 10 year rule, a QCD will be tax free, but there is no current year tax benefit. However, the account balance will be reduced by the QCD and therefore future taxes would be reduced.
3) Yes, the 100k QCD limit applies to all QCDs whether from owned or inherited IRA accounts, ie. to all QCDs reported under the same SSN.
4) QCDs from these various IRA types are allowed as long as the plan is not “on going” and therefore did not receive any contributions for the tax year. QCDs can be made without regard to IRA type, but the total limit is 100k.