Magento – How to disable quantity and in-stock options for Virtual Products

Not sure if this fits your needs but here goes:
Like you said set the actual product will be the one with the lowest weight. (Sage 25g). This is qty = 1(or you can consider 1 qty = 1g and set the product to be available for order only in increments of 25.) If the price for 50g = 2 * price of 25g , the price for is 100g = 4 * price of 25g... and so on, then I think the problem is solved. Selling 50g is the same as selling 2*25g even if you pack it differently.
If price for 50g != 2 * price of 25g, then you can use the tier prices.
Set a base price for 25g (or 1g if you considet 1qty = 1g). Then add the tier prices: for qty = 2 (or 50) price = X for qty = 3 (or 75) price = Y.

In this approach you don't need bundle or configurable products.

As per the updates in the original question, we found the below solution (workaround?) to provide the best way of achieving what we wanted. I think the only other way would have been to either create a completely new product type for these or implement some condition based extending of the configurable and grouped product types...

Further testing and the following scenario works:-

Retail Store

• Parent Product (00X) <-- This is a configurable
  • Child Product (00A)
  • Child Product (00A)
  • Child Product (00A)

Wholesale Store

• Parent Product (00Y) <-- This is a grouped
  • Child Product (00A)
  • Child Product (00A)
  • Child Product (00A)

The three child products under the configurable and the grouped are the same products. This means that only the parent product will have to be duplicated in the catalog (instead of every product).

• A parent product (configurable) for the retail store.
• A parent product (grouped) for the wholesale store.