Hi @dima,
both ftrack-sftp-accessor and ftrack-s3-accessor are api plugins and not server ones, therefore there's no point in trying to have them installed on the self hosted server.
In each repository you can find a snippet on how to initialise and use them just through the api:
s3-example , sftp-example
You can find an example use of the s3-accessor in this user-location plugin
for your third point, there's no need of using lambdas , you can just rely on the internal ftrack events to do so, please check the user-location above as reference.
hope it helps/
L.