(1) not sure
(2) Yahoo.co.jp is still a thing but I'd say Google is still the main one
(3) if you'll be residing in Japan it should be easy to get a normal phone plan. iPhones are dominant. I think it's all eSIMs now.
(4) I haven't used it myself but you can try
貸店舗・貸事務所・月極駐車場を探す 【goo住宅・不動産】
(5) I'd check out the various Facebook groups as well as Facebook Marketplace