The first 3 ideas there are good, the only problem I would see is that her mum will get suspicious of why she can never come to see the house, especially so if used more than once.
Option 4 though,
outright telling the truth does have huge potential to go very badly depending on mums ideology however, might also be surprisingly easy. We often assume the worst will happen but a good parent only wants the best for their child, and for them to be successful. It may be the case that opening up might not be understood but could be accepted, but telling a parent about being AB is a bit more that just telling your mum "hey I like to wear diapers", it's a lot more to explain and harder to understand.
In this instance though there is the fact that Pril is obviously doing quite well enough in life to have moved far from home and have her own place that she keeps up with (helps if the rest of the house is clean, tidy and well managed), and also well enough to have a well stocked AB nursery (not exactly cheap). I don't know how well she is doing in other ways (work, education etc.) but that would all help too.
Telling her mum "hey before we have a tour of the house I have something important to tell you and please hear out my explanation..." and then walking her through some AB info might not have the devastating result expected, it may be difficult to explain things and may be an awkward or rough talk to have depending on how open mum is but either way it would solve the problem of having to hide everything whenever she comes round, she should understand it is your life you are an adult (even if you don't want to act so sometimes) and able to make your own decisions and live how you choose.
If it was me and I thought it would be a fairly common (like 3 or 4+ times a year) occurrence for mum to visit I would probably go with telling her, but I know my mum is of the accepting and understanding mindset that would take it in her stride, Otherwise I would just look for ways to make it easier to hide stuff thinking she doesn't really need to know.