Why not buy one decent pen “shell” and then just buy the plastic tube with the tip and the ink afterwards?
I know many companies use pens for marketing but still, they could apply this too and also stand up for the environment while still do marketing.
Google “waterproof fountain pen ink”. Here’s a comparison of 137 waterproof black inks.
India ink is for drawing, not for fountain pens. You need a specialised indiagraph to use it.
I’ve had multiple pens in my bag, the worst leak dirtied the nib a bit.