1. If you follow all 3 steps from https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/translate/#loco_translation you should be able to translate all string. Let me know after following this. You can upload your translating in https://translate.wordpress.org/projects/wp-plugins/front-end-pm/ so that other user can use that, also your name will be there in contributor list.
2. https://www.shamimsplugins.com/support/topic/from-name-in-admin/#post-15204
3. https://www.shamimsplugins.com/support/topic/remove-setting/#post-12519
4. https://www.shamimsplugins.com/support/topic/show-name-of-members-in-the-conversation-not-the-person-who-made-the-last-reply/#post-19088