They-plural..so ‘were’ Person- singular..so ‘is’

The difference is mainly about whether we are talking about a past situation or a person who exists in the situation now. In “That signal would mean that they were coming,” the speaker is imagining what the signal meant at that time. Because the main idea is viewed from a past point of view, “were coming” is natural. For example: “We saw a light in the distance. That signal would mean that they were coming.” Here, we are looking at the situation from a point in the past. In “What would you say to a person who is making a claim?” the word “would” does not make the whole sentence past tense. It creates an imagined situation. The person is making the claim at the imagined present moment, so “is making” is natural. For example: “What would you say to someone who is asking for your advice?” We use “is asking” because we imagine the person asking the question now. So, “would” does not automatically require a past tense. The tense after it depends on the time and situation we are describing.

I am/was You are/were He, She, It is/ was Singular person/noun is/was They are/were We are/were Plural person/noun is/was

Hi "they" requires a plural verb (were) "a person" is a third person singular. Therefore, you need "is

Subtle. Once we are in the ’would’ scenario we maintain the conjecture and possibility and ambiguity and suggestion..by using the two former terms. The is breaks the hypothesizing mold and makes it real. That’s my thought. But I grant that the difference might not be hard and fast or based on a rule. Poetic license, choice of expression maybe.