Where the Mask Ends ending explained

Short answer: it's a happy ending

The real heiress gets her name back, wins over Lu Zhuo, and the two end up together and ride out to the frontier as a couple.

What the official synopsis says

Before filming, the production filed a full-story synopsis with China's National Radio and Television Administration. It is the most reliable spoiler available because the producers wrote it. In our own words:

Wei Rao is the true daughter of the Wei household (the Chengan Earl's house), swapped out as a child and raised poor.
The girl raised in her place is so cruel that her fiancé Lu Zhuo dislikes her. When he is wounded and unconscious, she refuses to marry him, and the real Wei Rao marries him for the money her mother's treatment needs.
Lu Zhuo wakes and is gradually won over by her honesty. She gets the Lu family through several crises and earns their respect.
The fake Wei Rao keeps trying to expose her. Wei Rao and Lu Zhuo investigate together, the truth of her birth comes out, and the lovers end up together.
War breaks out on the border. Lu Zhuo leads the army, Wei Rao goes with him, and together they defeat local strongmen, settle refugees and keep the region at peace.

NRTA production filing synopsis (via Baidu Baike)

What the first 10 episodes already reveal

Wei Rao's mother is in the palace. Zhou Fu, now Consort Li and then Zhaoyi, is Wei Rao's birth mother. She meets A Chou in episode 10 and doesn't recognize her.
The swap was planned. In episode 2, Guo Yufeng checks an old governess's room for something buried years ago, the first hint that the babies were switched on purpose.
Lu Zhuo knows his wife. When Jun Dai is presented to him as his wife in episode 11, he sees through her at once.

Does the drama's ending match the novel?

Not exactly. HK01 notes that readers of Xiao Jia Ren's novel see the drama as a rewrite: the book is a slow-burn, marriage-first romance, while the drama adds the real and fake heiress feud, court politics and the border war. HK01 preview

One Sina article says that in the novel Wei Rao rises to become a grand princess and pacifies the frontier with Lu Zhuo, and that Jun Dai comes to ruin. It's an aggregator piece that mixes book and drama details, so we list it as not confirmed. Sina Entertainment, novel ending article

FAQ

Does Where the Mask Ends have a happy ending?

Yes. According to the official production synopsis, Wei Rao's identity is proven, she and Lu Zhuo end up together, and they go to the border side by side.

Do Wei Rao and Lu Zhuo end up together?

Yes. The official synopsis says Lu Zhuo is won over by her sincerity, they investigate her birth together, and the lovers end up together.

Who is the real Wei Rao?

A Chou. The official synopsis says she was swapped out of the Wei household as a child. The girl raised as Wei Rao is renamed Jun Dai after the wedding.

Who is Wei Rao's mother?

Zhou Fu, the Emperor's favorite consort (Consort Li), played by Mao Lin Lin. She meets A Chou in episode 10 without recognizing her.

What happens to Jun Dai?

The official synopsis only says she keeps trying to expose Wei Rao. A Sina article about the novel says she comes to ruin; that source is an aggregator, so treat it as unconfirmed.