Old Hunter Henriett & the Witch of Hemwick

I have been ignoring Vicar Amelia for some time as I went to Old Yharnam and Hemwick Charnel Lane instead. I have opened both locked gates in Cathedral Ward, allowing me to summon Henriett:

However, I have heard rumours she vanishes after defeating the Witch Of Hemwick.

Is there any truth to the matter?

I do know I need Insight to summon her (I currently have 12 before the Witch of Hemwick boss fight) and have a few Madman's Knowledge stashed just in case…

Once requirements are met (wearing specific rune, doing something - in your case you've already opened gates), summons in location are available until boss of that location is defeated. For Old Hunter Henriett this location boss is Vicar Amelia and not Witch of Hemwick.

In DLC location summoning Confederate hunters will prevent one from summoning Henriett, but for Vicar Amelia there are no other summons.

And don't worry about insight, you need only 1 to summon.