How was "Splinter" the first word of the Turtles?
 

Minor problem but how exactly did they know how to say it? The fact that Splinter was surprised by it implied he wasn't able to speak at the time, so they didn't get it from him. If that's just something they called him at random, then it wasn't really his name as he implied it already was. It's certainly not something they overheard a human call him, in that case it would something like "Holy shit-eating Christ! That's the biggest fucking rat I have ever seen!".

I don't think it's ever been implied that Splinter was already his name in the Mirage comics. As for how the turtles learned words it's presumably from picking them up being around humans before Chet lost them.