Diane's scene was nuked when they removed her house for the lolbarn (cause reasons). Erik's telescope was deleted cause it's hot garbage (like most of his route) > once he gets a remake, you might get it back (and improved), though depending on what splittage they go with, Tammy might not be dickable by Erik anymore .....
There is no *need* apart from what others said (Diane, Maria). It's a fetish like anything else. However, my mod lets you bang a couple of pregnant girls, not just Jenny and Maria, if you are into that.
Basically you can do whatever you want in any order. But some stories do overlap. If you want to complete Diane's story, be prepared to do some of Jenny's and Debbie's as well, plus Main quest. It also depends on how far other characters are in their story if you are meeting with them.
You basically described every other preg for every other girl ....... There "might" be a shopgirl remake soon™ with most of the mall girls getting lewds, maybe even preg stuff. But that's a huge "what if" and at least 3+ years
Right now her scenes are just really, really old and she doesn't look particularly sexy in them, either - especially compared to let's say Becca, Grace or more so Tina.
A few theories :
She is rated enough and her prices are indicated in the massage pamphlet. She is just a prostitute and this attention is quite enough for her.
