How do I enable editing on a smart object?

You can also add a blank layer above your smart object layer and use the healing brush on that with the option of either "Current and Below" or "All Layers" selected. However, if you make any changes to your smart objects, those edits with the healing brush won't match your altered smart object.

Smart Object  Layers pixels can not be changed with Photoshop tools like Photoshop healing brush.  You should be able to use Adobe's ACR Filter as a smart filter on a smart object layer and use its spot removal brush.  The smart object layers pixels will not be changed  however, the layers composite  will be altered by the filter. I would think that Lightroom develop module would also have a spot removal tool. You can also open a Camera RAW File in Photoshop though ACR as a smart object layer. Editing the Layer object will open the object in ACR and you can work on the RAW files camera raw data in ACR not on a converted RGB image.