Re: Uchideshi in Iwama
I was based at the Ibaraki Shibu Dojo (the "Iwama Dojo" and Aiki-Shrine) for a while and I still go back to train there regularly. It is still part of the Aikikai and run by acting dojocho Isoyama Shihan. No problem with kyu grades or even absolute beginners in my experience. Check out the website for details. Inagaki Shihan usually handles admissions.
I never trained at Saito Hitohiro's dojo but I know a few people who do. It's maybe another five minutes further out from the station (although the accommodation is actually right next to the Iwama Dojo and Aiki Shrine). Entry details are on their website.
You might also want to look into Nemoto Sensei's "Aiki-house".