Hello,
Once installed on the Ledger, can we still use the Hive Keychain application to validate message, view, article, transfer, or are we obliged in the future to always go through the validation of the Ledger?

You say it's not free accounts, but if I read in ecency the terms:

Fees for using the Services
10.1. Account creation fees. We currently do not charge a fee for creating accounts, but we reserve the right to do so in the future, and in such case any applicable fees will be displayed before you use any service to which a fee is charged. apply.

If you replace all of your keys (owner, active and posting), you won't be able to use keychain anymore right now. Every operation will need to be signed with the Ledger device. You can of course leave your old active and posting key, just replace owner to be safe and keep the possibility to use Hive Keychain. You will be asked which keys you want to replace during the onboarding at hiveledger.io. You can also later change your keys but right not it's a bit limited. We're working on a better way to manage keys and to add Ledger support into Hive Keychain. It should be available in a few weeks hopefully. Until that time I recommend replacing only owner key.

You say it's not free accounts, but if I read in ecency the terms:

Ecency basically pay the fee to create your account. You can consider it as a business investment to gather new users.