The Iver Huitfelt ships are pretty big to be honest, they’re almost the size of a 45 and definitely more seaworthy than a 23. And if a 23 can escort people okay then I’m sure the arrowhead can.
As for modular weaponary, it’s the in thing. They love sticking Phalanx on stuff because all it really needs is a power supply and a control desk. Sadly the LPDs have lost their goalkeepers for Phalanx, for this reason, so I’d say that if it was required to arm merchant ships they’d just crane a Phalanx on, probably ex-Centurion ones at a guess.