The two blank lines (line-breaks) that appear between the revealed "Inspect the bathtub" link and the existing "Kitchen" link are a result of two things:
1. The first blank line is the one that you manually placed between the line containing the "scurtain" Named Hook and the line containing the "Pull the shower curtain" link. Any contents you inject into the Named Hook will appear above that manaually added blank line.
2. The second blank line is the line where the "Pull the shower curtain" link appeared, while the link itself disappears when it is selected the line it is on does not.
The following example demostrates this effect, you will see a blank line after the link is selected.
Some text
(link: "Selected link will disappear, the line it's on wont.")[]
Some more text
There are a number of ways you can resolve your issue, the followin solution uses a combination of the (link:) macro's self replacement functionallity and Collapsing whitespace markup to force the "Inspect the bathtub" to appear where the "Pull the shower curtain" link was.
|scurtain>[The shower curtain is drawn in front of the bathtub. You know that there's nothing behind it, but it still fills you with unease.]
(link: "Pull the shower curtain")[{
(replace: ?scurtain)[There is nothing behind the curtain. There never is.]
[[Inspect the bathtub|Bathtub]]
}]
[[Kitchen]]