What is the point of the girls in the mansion?
Nora and Danielle were added as rewards for "X-tester" ($30 tier) patrons.
Can the girls get pregnant?
Currently no. Dev has indicated it may be added at some point, but not soon.
Screens where the girls have expanded abdomens are related to the "hollow princess" system, which involves placing a girl's "soul crystal" into another's body (presumably their womb) to corrupt it; IMO this is more akin to oviposition. Also it doesn't seem like it causes the breast development a pregnancy does.
What do extractor pools statis tank milk machines do?
Extractor pools: Insert a target to generate lots of "red crystals" that don't have a use yet. In the latest Patreon versions, the machine can be configured to extract the target's "soul crystal" for the mechanic described above.
Stasis Tank: Doesn't do anything yet but hold a target in place and give you another way to stare at them naked.
Auto-trainer, Milker configuration: Continuously builds an installed target's arousal and generates a milk item every time they orgasm. The milk has no use yet, but comes in different grades (S, A, B, C, ...) roughly correlated with the size of the target's breasts.