There is a huge summoning ring on Demonreach, it was shown in Cold Days.
If you know the true names of the being you are targeting you just summon them.
Ethnui obviously isn’t the Titans True Name and Harry didn’t have decades to find it out and compose a summoning ceremony. He could do a binding with the name Ethnui with the aid of the Spear and at Demonreach’s ultimate extension.
The other point is we don’t know what the Crown of Thorns can do, I have posited that it is a god level set of thorn manacles. If Harry can figure out what it can do (and it surprised me he had figured out the placard) and it is what I have posited, then Harry can go out and use it to depower potential foes, to transport them to Demonreach, whip it off and put them down, as an alternative to creating a summoning.
Harry is actually good at Summoning, Mother Winter, The Summer Queen, and the Winter Lady were all summoned, as was the Goblin King. He didn’t think he stood a chance summoning Ethnui, so didn’t try. He disn’t Have time to ascertain what he needed.
I suspect though after taking some one down he can control the released and summon them back easily via the summoning circle should he need to. Ethnui is is crude prison parlance Harry’s “Bitch”.
Summoning Marcone/Namshiel Jim has given us clues all along -in his description of the character, a stack of dollar bills for the colour of his eyes, a luxury yachting cap for the yachting tan (preferably owned by Marone), an expensively tailored suit made by Marcone’s tailor ro Marcone’s measurements , a well balanced throwing knife used to kill, a tiger for its soul and Helen Beckitt’s daughter at the centre. Harry knows this after the soul gaze and investigation of Marcone over many years.
Ever wonder why Jim repeats certain things in Harry’s descriptions of characters? I bet it is actually Harry’s subconscious making the assessment for summoning and binding. His subconscious is much smarter than his conscious.
Marcone isn’t Marcone’s real name, when Harry finds that out he can summon him, with everything he knows about Marcone, and with him Namshiel, from Demonreach.