I realized the npc can be killed by projectiles, which feels like something I do not wanna happen.

Maybe you can add an invulnerability option in the interface so that nobody can accidentally kill the npcs.

If invulnerability is very problematic coding wise just make the invulnerability option add an infinite level 255 resistance effect, not technically invincible but might as well be. That is what I'm doing for now for the npc.

For anyone who wants to do that effect too just do the "/effect give" command while looking at the npc. The string of numbers & letters is the npc.

Thanks for the report. Could you please confirm if this happens in "creative" mode or also in "survival" mode ?

All NPC are automatically invulnerable by default, see:

Unfortunately they can be still hurt like any other entity in "creative" mode or by "creative" mode players.
If this happens in creative mode, it's by design but I'm happy to add something to ignore the damage in this case (which is easy to implement). ;)

Thanks for the confirmation.

Oh yeah, it is only in creative mode! I didn't realize that creative mode players can target them differently than survival mode, I just assumed damaging is something that isn't related to what mode you are in. So I guess the problem never really existed.