Should you have to declare yourself trans to crossdress?
No, I don't think so. This is a weird one, though, because I don't think anyone thinks you need to be trans to crossdress. Seems like most people think everyone should be allowed to or no one should be allowed to, I'm not seeing the constituency for "only trans can cross dress".
I'm in the "everyone can cross dress" camp.
To use different pronouns?
Except for kings and queens (and people with pretentions to same), nobody actually uses pronouns for themselves. People request that
other people use pronouns when referring to that person.
Pronoun preference is a matter of courtesy, it should never be a matter of law. And that applies on both sides, meaning the person making the request and the person deciding to grant the request. I think it's rude to request the use of non-standard pronouns. I think it's also rude to request the use of pronouns that don't match how one presents oneself. So my standard is correlated with trans status, but it's not identical to trans status.
To us a specific bathroom?
I'm not really sure about why you're asking the question. The problem doesn't seem to be that males are going into female bathrooms without declaring themselves to be trans. The problem seems to be males who DO declare themselves to be trans, but still present as male, going into female bathrooms. It's not the declaration/non-declaration of trans status that's causing friction.
If you pass as female, use the female bathroom. If you don't, use the male bathroom. Note that passing is a higher standard than presenting.
So real world hypothetical. I'm a biological male in a men's public bathroom. I'm biological woman who identifies as a man comes in. Don't ask how I know just run with it. I don't kick them out of "my" space. Then I biological woman who DOESN'T identify as a man comes in. Again I don't how I would know, sorta both the point and not the point at this juncture but just go with it.
Would be transphobic to NOT ask the biological woman who doesn't identify as a man to leave?
In that scenario, you aren't upholding sex segregation. That's not transphobic, but it's also not trans inclusive. It's just... odd.
The trans activist position is that we should maintain sex segregation, but provide an exception for all trans people. The opposing position (which gets labelled as transphobic but that's not quite the right term) is that we should maintain sex segregation but no exceptions should ever be made for trans people. Middle ground positions are that exceptions can be made for trans people who meet certain criteria. And the true transphobe position is that there shouldn't even be any trans people.
The position that we should just abolish sex segregation entirely (and as a side effect make trans status irrelevant) is orthogonal to all of those positions.
And this isn't a joke or a trap question or a setup for a gotcha. My biggest issue with has always been if you don't treat the biological sexes differently the whole concept of "trans" gives you nothing to do, it's meaningless.
That's absolutely true. Without sex segregation of some sort, trans status is irrelevant.
But almost nobody wants to do that, including the trans activists. And there would be serious consequences if we did. It's very much a Chesterton's fence.