No. The app does not do any changes to the store theme.
No. The app does not have any impact on store speed.
No. Shopify does not allow individual variants to be hidden. They allow entire product to be hidden.
Yes, You can create Hider and Publisher in the app to run at your given time to hide and publish products.
To make a product visible for the duration 9am to 6pm.
STEP 1 - Create a publisher by scheduling the time as 9am. So that the products are published and are visible for the customer.
STEP 2 - Create a hider by scheduling the time as 6am. So that the products are hidden at 6pm and are not visible for the customers.
By following STEP 1 and STEP 2 simultaneously as shown in the above image, You can make a product visible from 9am to 6pm.
Just by removing filters you can select all the products. You can see in the below snapshot.
All the products by default have variants i.e., Product details itself are considered as a variant. Therefore 'Product Variants' filter will include products both with variants and without variants.