at 50 cm you have big pachypus: in nature the plants that I saw with Brian Bates were smaller
I think the main problem is that we do not suffer enough pachypus in our latitudes and incidentally the lack of brightness

ACM should be watered all year long otherwise stems dry
For the roots try the rooting hormones or fertilizer rooting10-52-10

hookeri susannae ?

with 2 heads it's not common for this specy

capitata ?

yes a cristate form of Mammillaria compressa
Can you show us the whole plant ?

it'a an hybrid of Aloe rauhii or deltoideodonta but there are so many hybrids right now that it's hard to put a name on your plant

look like Haworthia cymbiformis 'Frogmouth' ?

an hybrid of brevifolia or humilis ??

Iann it's not a single plant but many with 2 heads