Lactose will give it a milky taste and sweetness, which I'm guessing pairs with the 'buttery flavours. I don't see anything 'wrong' with it perse.
Any reason it has to be a mead if you're going for a butter rum flavour? Could you try an ale of some sort? Is it carbonated? I've never had it.
Also, my suggestion would be to brew a bigger batch, say a gallon. Then pull smaller sizes. Say 1/4 gallon at a time. (I'd do 100ml and make it a 4l batch but I'm metric) then dial in your ratios. That way you can scale it up easily?
If you're having trouble adding the concentrate in such tiny volumes, you can also dilute a bottle of it with mead, say go to 10x ratio (1oz bottle with 9oz to get 10oz) and then you have 1/10th the strength and can add it a little easier. E.G you know you need 20 drops of rum and 10 of butter at 1/10th then you know you need 2 and 1 of full strength.
Just some ideas, not sure how strong or what size your concentrates are.