This is definitely a painful experience. Will you be able to swap it out with the seller? The Metropolitan is a pen that just has to work out of the box without any tinkering.

Emerald of Chivor!

The Cavalier should have a good cap seal - so there's no danger of it drying out if left unused for a few weeks. But, even if you find it has dried up, there is nothing to fear - just flush it out with water till the water runs clear, shake it (like you would an old-time thermometer) to remove any excess water, and let it air dry. Then you can ink it up and you're good to go.

Hobonichi notebooks and planners are all Tomoe River. So are Endless Recorder notebooks. Galen also has notebooks in TR. Now is a good time to get TR - it is still available in many places :)

As long as you don't let the ink dry in your fountain pen, you should be fine. In a fountain pen with a properly sealed cap, that should easily be several months. I recently opened a Pilot V pen that I hadn't touched since 2020 and it wrote on the first try.

If you can share what pen you are using, maybe we can give you a better answer than "It depends" :)

Tomoe River 52 remains a top favourite - I have a couple of notebooks left, so should have it for a while. Have a couple of TR68 notebooks as well.

Cosmo Air Light is a close second - brings out ink properties beautifully and can take a lot of abuse.

The best value for money I've found though has been the Nakabayashi Logical Prime notebooks. The paper is fantastic, and the notebooks are priced decently.