Investors & landlords

Question on de minimis safe harbor:
1. Can this be applied to appliances(like washer/dryer/microwave/range hood), furniture, repairs/maintenance, cleaning, electronics ?
2. Can this be applied to items not 100% used for biz ? Example: Appliances/furniture/electronics shared(say 50/50 equally) between renters and owners.
3. Can this be applied to expenses incurred in a prior year where there were no rental activities until the following year ? Example: Expenses incurred in 2017 and rental activities didn't start until 2018.
4. Can this be applied to above-mentioned 1/2/3 all-combined ?