Hoo, boy! Ms. Zen was, to me, way out of line. Taking over the house? We are Buddhists, not Zen, and we don't know any Buddhists, including Zen practitioners, who would do that! We just pretty much want a quiet and private space to do our practices.
We do a twice a day practice, and we can do it anywhere quiet, where our vocalizing won't disturb others (we can even whisper under our breaths, if we must use a "Cone of Silence" in a public place, but we WILL do it.) We use candles and incense on our altars. Many of us use electric candles for a variety of reasons -- fire concern certainly being one. If one cannot use incense, even the smokeless kind, for any reason (allergy, illness, ran out, etc.), well, then, one does not! If he is a Buddhist, he may be well in the clear, in terms of correct practice, without real candles or any kind of incense.
However -- an altar? If he is Buddhist, that may be important or even central if it contains a sacred object used as a focus or locus of practice. I could easily do without candles, incense or a bell (we ring bells as part of the practice) but the altar? That is very basic. If I am traveling or in a hotel I may have to do without an altar, but somewhere I lived? The first question one asks one's self is, "Ok, so where does the altar go?"
I suggest OP needs to think about her family's comfort level with other religions (we extend the same courtesy to others that we would like when visiting -- quiet and private time to do whatever they do) in their household. If OP can't get behind an altar, and his practice really requires it over the long term, perhaps he could talk to his priest and make arrangements to go to the local temple regularly, or, if that is far, to someone's house for daily practice as often as he can. We would set a schedule and have someone in Kyle's situation over every day, and I think most other Buddhists would also, if they could.