to get Ella in Berlin you need to get Vicky on board with it. Vicky's face was dead-set against the match IIRC. She even wrote to her mom saying she didn't want the match to go ahead, suggesting everyone from her husband's Baden nephew to the duke of Manchester's heir, to avoid the wedding. I'm not sure what Vicky's objection was, I doubt it was the hemophilia since neither Irene nor Alix were married yet, and Vicky had no objections with Irene IIRC, so I'm assuming it was personal.
Another thing you're going to need is for Ella to be as "into" Willy as he was into her. Or at least "interested". OTL, she wasn't ever interested in Willy, despite both his grandmothers being keen on the match, and she later confessed that his "ardour" had scared her somewhat