Can anything harmful be done using the SDK if someone was able to obtain the app_token and customer_id?

Users cannot initiate malicious requests just by having access to the customer id or app token. To complete a pay request for instance, the SDK requires a payment intent id which has to be created via a secure MTLS call or using OAuth (OAuth is a standard authentication method used to secure APIs and other infrastructure using Public and Private secrets combined with short lived access tokens).