The other tenant still has legal possession of the unit - they signed for it.

Buying a gun is not essential and your not violating anyone's rights by not being open due to a pandemic.The fact you own a bunch of guns and have a concealed weapon permit is the exact reason why in a situation of crisis like now, when people feel threatened, they feel the need to possess a gun.

The detainer filing will cause a snowball of grief on them and unless they can prove to a judges satisfaction that they have rights to adverse possession, they may be arrested for criminal trespassing (penal code 602 PC) as they are removed. 

Get a local attorney familiar with this type of adverse possession.

Under Insurance "tenant to provide own insurance for possessions both inside and outside of premises.   

"the bill would also authorize a defendant, for any residential unlawful detainer action that includes a cause of action for a person continuing in possession without permission of their landlord, to notify the court of the defendant’s desire to stipulate to the entry of an order.

According to WA law (RCW 61.24.060), "The purchaser at the trustee's sale shall be entitled to possession of the property on the twentieth day following the sale."

Don’t let them take possession.

There are lot's of options so you need to determine your skills and passion and find a possession that suits both.

That is the only fair thing, to let them protect their initial investment by paying extra money to redeem from the tax sale, even if it is an earlier investor redeeming from a later auction when they failed to pay the taxes.You protect yourself the same way you protect yourself from former owners or their heirs redeeming--you establish exclusive possession, you burn off redemption rights, you buy out people's rights, you engage in strategic litigation to eliminate rights, etc.