Will they call the police? They should call the police. And have Watanabe live with her girlfriend/wife classmate for some time.
I know it's hard to stop stalkers, but having a crowd come intimidate two students, or people sharing someone's private information (like an address) should at least give them a fine, right? And these dudes living in their mom's basements surely won't have the money to pay for a fine, they already wasted it all on idol goods or whatever.
Point is: Police should get involved. ASAP
The mother's concerns are valid. The decision to force her daughter to terminate her contract with an agency given that this would mean breaching working contracts with several projects she's in is clearly motivated by her past marital problems and her apparent hatred for the industry, so it's bullshit.
First step after communicating the agency would be contacting the police. Then of course the agency could argue "there is too much drama so we best not give an official pronunciation about the stalking issue", even tho I believe this is absolutely shitty given that the girl is a minor, which just makes it all worse. A bunch of people (mostly adults) are intimidating, stalking, doxxing, etc a 17 yo girl and putting her in danger of more severe crimes. The agency should reprimand that because real fans don't do stuff like that. A "authorities have been warned" message would be enough to scare most of them off, it would also let these weirdos know that the agency is backing up the actress and she is not vulnerable for them to keep following. (Additionally, the agency should provide security for her. Somewhere or at least a plan to hide her or expose her a bit less until it has all been dealt with with the authorities)
I have no idea how idols and their agencies go about stalker fans, but outing someone's address is too messed up to not be brought publicly and for legal measures to be taken. And the haters can't argue that paparazzis do this with famous actors and it's not a huge scandal because, again, it's a minor. (wouldn't be any less wrong if she was of age, but it's more fucked up)