On b58 - ss58
I agree that transition should be as simple and transparent as possible. It is fine to use b58 keys like in v1. I also did a post to explain why ss58 is a good idea (in French, but code examples should be understandable Format de clé et vérification - #3 by HugoTrentesaux), and that’s why it’s the format used internally in the v2 tools (like duniter-squid use this as primary key for accounts).
Your suggestion of keeping b58 for old accounts and ss58 for new accounts seems reasonable. The only challenge I see is to persist it between tools. For example if people re-install Ğinkgo on an other phone and import their accounts, the address format should be unchanged. We could add a helper field server-side in duniter-squid like a boolean “existed_in_v1”.
That way, Cesium also can choose to display b58 instead of ss58 if the account existed in v1. Do you think it would fix the issue?
Other topics
You mean Cesium+ avatars? In which tools?
True. Even old key generation methods are still perfectly valid.