Do imported orders reduce my stock or send emails to customers?

No. Importing orders doesn't send any email or WhatsApp message to customers, and it doesn't take anything out of your stock. That makes it safe for past orders.

Each imported order gets a "Order created manually" entry in its Timeline.

One thing to watch: if you later open an imported order and change its status to Processing with Update Status, Wcart treats it as a live order and reserves its stock then. For historic orders, set the final status in the orderstatus column of your file, for example 4 for Delivered, so you don't need to change them afterwards.