IRR

It is only relevant when comparing projects. If comparing projects it is not valid to simply choose the one with the higher IRR (because the IRR is simply the rate of interest at which the NPV is zero) - we should choose the one with the highest NPV. However, if it were the case that inflows were to be continually re-invested at the IRR, then (and only then) the one with the highest IRR would be the best. It is really not until P4 that this becomes an issue, which is why there is something called the MIRR (modified IRR), but that it not examinable in Paper F9.

Yes, but that will never be the case in exam questions. The relevance for the exam is only for written parts of questions.