Does anyone know offhand whether 2018.2 SCA supports scripts written in SS 2.0? We have a number of SS2.0 scripts on the Sales order transaction that are causing errors in the add to cart and checkout process on the SCA side. SS1.0 scripts don't seem to cause a problem.

If by "scripts written in SS2.0" you mean user events and scriptable carts (client scripts on the sales order) then yes, by default SCA supports those. (and that is agnostic of SCA version because it's another layer of the system). That being said, there are some APIs and operations that are not compatible in client scripts (for example, anything on a client script that runs in scriptable cart and uses N/UI modules). And then there are user events patterns that don't play well with SCA either, but this is less frequent.