Hi!
Do MPs convert into credits or do they represent something else?
Manufacturing Points are the output of your factories, when you give them money as input. You can "convert" MPs into ships and buildings. If you don't build anything, are those MPs "lost", but you aren't charged for them. And you can't get more money back - those MPs aren't converted into more money. There's also a transfer of MPs from social production into military production, if your social queue is empty (you're not building any building).
How about TPs?
Technology Points are output of your research buildings. Even if you don't research anything (your tech tree is all done), you're still charged for them. So when that happens, you better convert all reseach buildings into something else.
BR, Iztok