Codex Advocare invocation switching: abusive or RAI?

Codex Advocare is a warlock item from Ravenloft. The relevant text:

Description: The book is bound in red leather. The title is stamped in silver foil in Infernal on the cover, and the interior is scribed with the descriptions for all manner of invocations, also written in Infernal.
Activation: The book must be formally claimed in order to gain the benefit. Instructions for claiming the book are printed clearly on the flyleaf. The book can only have one claimant at any one time - the most recent claimant becomes the sole claimant. To claim the book, you must read the invocation descriptions from beginning to end (a task of some 8 hours) and then verbally claim the book. Once it is claimed, the book always provides its benefit to its owner, as long as the book is part of its owner's equipment.
Effect: You know one more least invocation than normal (you must be able to use least invocations in order to be able to utilize this ability).
(Lore, Aura & Construction omitted)
Price: 20000 gp.

My optimization idea for this item has two parts:

1: A non-warlock who knows Infernal can claim the book, but they don't get any benefit from doing so.

2: Warlock claimant wants to change invocations. She gives the book to eligible friend. Friend claims the book then gives it back. Warlock reclaims it and selects any least invocation of her choice.

Result: 1 full day (8 hours reading, 8 hours reading, 8 hours rest) = floating invocation.

Interesting situation you've proposed.

I don't know if this was intended, but a floating Least invocation isn't powerful, and certainly not a troublesome as, say, a prepared caster changing their entire list of spells memorized on a daily basis, so I'd be inclined to reward someone who thought of this by letting them do it. You can, notably, do this exact same thing with a Warlock who has taken 2 levels of the Chameleon PRC, which grants a floating feat which can be used on Extra Invocation, and that has yet to cause any balance problems, as far as I am aware.

Probably because there's only about 6 least invocations worth taking; Eldritch Glaive, Baleful Utterance, See the Unseen, Entropic Warding, Otherwordly Whispers, and, at low levels, Spiderwalk and Summon Swarm. And of those, not all of them will work for every character concept.