Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
The game plan.
It seems there is a bug. Cooldown reduction should be sitting at 39%.
The desire of this Morgana is for pushing and heavy support. Early game is built around the concept of simply having enough mana to tide to over till the Shroud is reached and relying on spellvamp to keep the proverbial ball rolling.
The late game consists entirely of how well you can synergize your skillshot with other champion's snares and your shield timing. Primarily, your goal should be allowing opportunities for your team to score kills via well timed shields and skillshots.
As far as the purchase order, the shroud and boots are required to have enough tank to survive and have the capability to lock down enemy champions as much as possible. You can switch to morello's, over the DG, if your team has plenty of dps to take down tanks.
As team battles pick up in the endgame, an hourglass becomes required. Darting in the middle of a fray with a shield up to get off a team ult becomes common. Hourglassing as you become focused is a great way to survive until the second phase of your ult. A morgana that gets both phases of her ult off on an opposing team is often a deciding factor and worthy of lulz.
This is not a primary damage Morgana, but rather, a heavy support. Strong management of fast cooldowns brings excess shields, constant snares, and a lot of saved lives and opportunities for your team.