น่าสนใจมาก

Yes, but be careful. 'Kon née nâa-sǒn-jai mâak' means 'This person is very interesting (intellectually or as a character)'. It doesn't necessarily mean you are attracted to them, but it can imply it.

'nâa-sǒn-jai' is an adjective (interesting). 'sǒn-jai' is a verb (to be interested).

Yes, it is perfectly acceptable in formal writing, though you might use 'เป็นที่น่าสนใจ' (is of interest) for a more academic tone.

Just add 'mâi' (not) at the front: 'ไม่น่าสนใจ' (mâi nâa-sǒn-jai).

No, that's incorrect word order. 'mâak' always comes after the adjective.

As a noun, yes. But as a prefix to a verb, it always means 'worthy of'. They are written the same way but function differently.

Neither is 'better'. 'Dee' is more casual and slightly less intense. 'Mâak' is more emphatic.

It's often a polite way to acknowledge you've spoken without having to disagree or engage deeply. It's part of 'saving face'.

Yes! In texting, people often write 'น่าสนใจมากกกก' or 'น่าสนใจมากๆ' to show they are really excited.

You could use 'nít-nòi' (a little bit), as in 'น่าสนใจนิดหน่อย'.

Younger people might say 'น่าสน' (nâa-sǒn), shortening the phrase.

You can, but 'nâa-gin' (appetizing) is much more common for food.

Only if used sarcastically. Otherwise, it is 100% positive.

Focus on the falling tone of 'nâa' and 'mâak'. They are the most important for being understood.