Where to put standard functions that reference lib/mod methods

IMO, it's a sign of bad design when you start mixing procedural functions and OOP code in the way that you're trying to. If I ever needed anything similar, I'd just declare static methods in the class instead.

I kinda agree, but sometimes its easier to do something like...
echo "Your id is " . un_from_id; 
as opposed to 
echo "Your id is " . $this->Accounts_model->username_from_id('123'); 
every time. I thought that it was a no-no to add procedural functions, But ive seen it done quite a few times, including in WP, so oh well

I suggested static methods, you call those directly as className::methodName().

And you're going to see A LOT of crappy code from all kinds of sources, don't take that as a validation of bad practices ... especially WP is probably the worst source of inspiration in that regard.