Regarding MIRR, cash outflows are reinvested at the MARR until the end of the project - this agrees with what I said.
You say: "The Internal rate of return (IRR) assumes that cash flows from a project are reinvested at the IRR."
The IRR for a project can be calculated WITHOUT having the cash outflows reinvested. It is a common misunderstanding to think that they must be reinvested.
If they are reinvested at the IRR, then you get the same IRR result for the project. It would be very difficult to find such an investment to give the same rate of interest as the IRR.