Butter and Lesser are just different lines of the same morph (just like Bananan and Coral Glow). In a case where you 100% know the lineage then it is probably best to maintain that name but in a case like yours where the lineage is unknown you can call it which ever you want - Butter Pastel Pin or Lesser Pastel Pin. In the grand scheme of things it does not matter either way
Still tricky given how close Phantom and Mojave are. I am inclined to stand by Mojave Pin. If it really matters to you then the best thing to do would be to raise it up and breed to a known Mojave. If you get a BluEL then you will know it is a Mojave and if you get a Purple Passion then it is a Phantom