Dear Nanzy,
These circumstances play out surprisingly often, and unfortunately pursuing legal remedies tends to end up being rather costly. Therefore a depending on the value of the estate, much of it can be consumed by legal fees. For this reason, most cases do end up settling. It is difficult to say whether it is 'easy' to establish these types of cases. Generally I would say that undue influence, or lack of capacity are not especially hard to prove if you have the right evidence. Family provision applications are generally easier to make out. But between the three causes of action you have outlined above, you may have fairly good prospects of recovering an inheritance from the estate.