Can I select records from CRM's database for inviting to sign?
Yes, it is allowed to select any Contact, Account or User.
Can I merge information from CRM against Custom Fields in a selected HelloSign Template?
Yes, after realizing HelloSign Templates had been imported you can choose the module a given Template should relate to. Then you can setup the formula for merging on each Custom Field on subpanel under that Template. Finally, on selecting that Template on sending a Signature Request each Template's Custom Field automatically fetches respective information from CRM.
Can I force original Subject and Message from a given Template?
Yes, on configuring an imported HelloSign Template you can mark the checkbox "Locked Message?" which tells CRM those fields must be readonly.