There are two separate question regarding necromancy and the law. Necromancy uses a specific energy that is fundamentally different from the normal magic that wizards, fae etc use (and is what Blacks and presumably reds used). It is sorrupting by nature, and is generally frowned upon. A separate issue is the Legality of its use, as defined by the 5th Law. The Laws themselves have a certain amount of leeway and grey area in them by nature, which harry has used to get by in the past. His summoning Sue was very much Necromancy, it just fell into the loophole of not being targeted at a human, which was a pretty thin defense, but they let it fly in the face of the Kemmlerites and the Darkhallow as a nessesary evil. If a warlock showed up that had a pack of zombie dogs running around doing his bidding (and the council had the time to spare) I dont think there's much chance he'd keep his head for long. Its the same with killing via magic in self defence: its still very much a violation of the Law, but the circumstances allowed for the council to impose a lesser sentence.
Regarding the line between Necromancy and Ectomancy, Its a grey, grey area. Personally, I think some of what Mort has done, especialy in GS, crosses teh line, whihc is why he keeps his head down so much around the council. The line I would draw for a game would basically be a social one. Ectomancy allows you to talk to existing ghosts, probably to track them, and if they are willing you can apparently get them to loan you a certain amount of their skill/power/memory. Anything that forces them to act for you, consumes power directly from them, or results in a physical form like a zombie, materialized spirit (which only crazed ghosts do anyway), etc. would be crossing over into necromancy, if not actually violating the Law. If you keep up the need for a social interaction with the ghost wherein you convince them to help you based on their own choice, knowledge, and/or perspective), then I think you can duck under the Necromancy issue. Basically keep them in the relm of willing allies, not slaves.