Were original longrifles with carving and engraving typically decorated by the customer at home?
No. There is a common notion online that original rifles were bought plain and later decorated at home by the owner, but that is generally not accurate for rifles with proper carving and engraving. Some of the lowest end builds, like barn guns, may have had minor home touches such as simple painting, but true carving and engraving work was not typically done by the customer after purchase.