The idea that Inklings are made of ink is not true. There is a better explanation for why Inklings cannot swim, or fit through grating, and it is one that doesn’t raise more questions than it answers: The explanations given to us in-game!

These two scrolls from Splatoon 1 and 2 respectively explain that Inklings have internal organs and that all the ink in an inkling’s body is highly compressed inside an ink sac. The latter image shows an x-ray displaying that this is the case. They also explain that ink rapidly disintegrates when exposed to the air (so a body made of ink would be impossible).

The highly compressed ink sac explains their inability to swim, as it increased their body density to be greater than water, causing them to sink.

As the first scroll describes, Inklings have no bones, just like REAL squid and octopuses! This means they can fit through almost any opening. Here we see an octopus escaping a glass box through a tiny hole. This is how Inklings fit through grating.

These chromatophores in a squid’s skin allows it to change colour. Inklings change colour in the same way. They are squid after all.

Octopuses like this one, as well as squid and cuttlefish are also highly adept at using this colour change for a variety of different purposes, such as camoflague. Inklings are also able to hide among their ink in much the same way this mimic octopus hides against the sand.

These explanations all come from in-game information, and real world squid and octopus biology. Not a fan-made assumption that raises even more questions, confusion and general weirdness.