I’m having some troubles with Bellman Ford Algorithm in the following exercise

My guess is that at the end the algorithm should detect the negative cycle BD DF FG GB since B is decreasing at Vth (i.e. 7th) iteration. However changing the distance of the source does not make much sense to me. Is the source to be considered at fixed distance 0 or it can change as well as all the other vertices? Or is it correct to assume, as I did, that the algorithm will detect a negative cycle in the end?

Moreover, assuming to proceed with the computations, it is not clear to me the order I should follow. Being Vertex A not connected, should I go on relaxing from C (I guess not since it is not directly connected to B)? Or should I go on from D, which is the first vertex in alphabetic order directly connected to B? Thank you