First of all, please note that if you make your BOM products "sellable", this not only will show up the BOMs in Stock Central, but it'll also show all your BOMs in your front shop and will be available for your customers to purchase them directly. Is this what you want?
1st - I need all of my Product Parts and Raw Materials to show up in Stock Central, instead of just in Manufacturing Central. I concluded that for this to happen I need to make them "sellable". However, there isn't a bulk action to make all existing product parts/raw materials "sellable". What am I missing? Through ExportPro I've noticed that if a Product Part/Raw Material is sellable the regarding cell value is "1" instead of a black cell. Should I export everything, edit the sheet and import again?
The only tool to do this change in bulk is through our ATUM Export & Import PRO add-on as you've correctly noted. And that "sellable" column in the database is stored as an integer value. The allowed values are: 0 (disabled), 1 (enabled) or NULL/blank (global).
But by default, all the BOM products have the "sellable" option set to global. That means that it'll get the value set in ATUM Settings >> Product Levels >> Add selling of BOM. So, if you just enable that global option, it should make all your BOM products sellable, that's all.
2nd - Being a campervan conversion business, we have each of the vehicles we convert listed in Stock Central. Within each vehicle page we've aggregated every item needed to convert them (BOM). Is there a way for us to Export the BOM regarding one vehicle at a time? I can only seem to be able to Export the full list of items, without specifying which ones belong on each vehicle (product). Basically what I need is an Export which outcome is - "Vehicle X: Respective BOM. When I select the Linked BOM column, every item shows up with the denomination of the vehicle (product) they're attached to.
If you want to only export the items linked to a specific product, you can filter the results by product ID (for example). More info: