Another interpretation of the "bad Latin" effect might be that spells are a bit like cooking. The exact quantities of the ingredients are not too important as long as there's some sort of balance, and you can leave out some ingredients entirely, but others are important.

So the creator of a spell starts with a complex 'recipe' for a spell with a long phrase of magic words and a complex wand-waving motion. Then the work of spell-creation is to reduce this into something memorable and usable by first year students. The outcome wouldn't have to be unique, just balanced, and will therefore be influenced by the style and preferences of the creator. Since early spells might have actually been named in real Latin, that style could have been retained to match the expectations of the wider wizarding world, where people thought that spells should sound that way. Perhaps the same spell might be invented several times, and it's only the most fashionable name which becomes common and taught to students.

This now gives me the silly idea that maybe in the Harry Potter universe, there could be a trend for Hipster-Wizards who recreate spells from scratch using different or 'ironic' magic words - who wants a wand-flick with the words "Wingardium Leviosa" if you can hold your wand between your Fedora and your beard and say "Cornflakes" instead?