Will Enabling multiple locations be the right choice for my store?

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I'm reluctant to make the change as it's irreversible. Makes me a little scared I was hoping to get some insight into what will actually happen if I enable it. 

I got one store, with 5 active fulfillment locations. One for my own warehouse (3PL managed with a private app to their WMS), two Dropshipping locations, one for vendors that don't provide stock, and one for a small local warehouse. 

My problem is that I can have one product stocked in 3 of the above locations adding up to let's say 5 in total (2, 2, 1, 0, 0). A customer gets 5 on the Storefront and places an order for all 5. I now want to automatically split that order into all 3 fulfillment locations that correspond to the inventory. Is this what will happen if I enable Multiple Locations?

I also have another problem and that is that my default location in Shopify is my own warehouse and 3PL. So any order that isn't unfulfillable will default into that default location but have I have no products available. Can I change this somehow so the 3PL doesn't end up with all my rest-orders? 
Due to one of my suppliers doesn't provide any stock, I have added their product but doesn't track stock in Shopify. Sometimes I end up with products out of stock and I have to cancel the order (but as it has defaulted into my 3PL, they still try to fulfill it (it's dormant in their WMS, due to not allowing a downloaded order to be deleted).