You should definitely add long vowel marks, otherwise I have to look up every word on Wiktionary to see how it's supposed to be pronounced.
Sculpting a V is much easier that a U, that's why you'd see Marcvs on a stone monument.
V is not just how a U was written, but is the capital form for the letter U, so words like Vranus and Vnum, Vtilitas, Vxor etc. would be found at the start of a sentence.
Nice one, Duolingo. I wilk be able to cast spells and enchantments. Thanks for the latin