Summoning a ghost and making it do something is a no-no. That's lawbreaking stuff (the stuff that the Wardens warned Mort not to do).
Opening the way for ghost to do what the ghost WANTS to do, that's different.
That's what Harry did in one of the books - allowed the ghosts to manifest so they could complete the task that held them there. Now if they had stuck around to tell their spouses "I love you" one last time then Harry wouldn't have gained much by doing that, but since they were sticking around to avenge themselves on their murderers they went after Harry's enemies for the win. Notice how a certain Duke (who was visiting and hadn't killed many people in Chicago) escaped that situation with his life - that's because there weren't a lot of ghosts gunning for him. It would have made things simpler for Harry if all the Red Court witnesses had died that night (no one to point the finger or tell their side of the story) but would have been Harry commanding the ghosts and he couldn't afford to pick up the Lawbreaker stunt just then.
Spoilers for Ghost Story:
Sir Stu is a special case. He is a slightly more self aware ghost and his 'thing to do before passing on' is to protect and safeguard his family. That means when it comes to protecting Mort, Sir Stu can do more than the average ghost can without someone needing to command him to do it.
When it comes to manifesting to defend someone, not even Sir Stu could manifest to defend Mort from that hitman. A ghost has to be nuts before it can manifest - so for the PC to get his ghostly bodyguards he would have to cast a rite to make them manifest - like the one that Harry did.
So how did Harry do that bit at the end of book 3? I'm thinking Thaumaturgy - and explaining Harry's ability to it with no prep as Harry spending his remaining FATE chips to tag aspects, using Lore to make declarations about the ghosts (giving him free Aspects to tag), and possibly taking a consequence or two. Then casting a zone wide "All Ghost Can Manifest And Do Whatever They Want" rite.
Richard