SCOTUS has expanded and contracted in the past, but has been at 9 justices since 1869. They absolutely can try to “pack the court” if they have the votes. I think it’s in poor taste, but they’d have every right to do so.
Congress can admit PR as well, but historically PR would have to draft a Constitution, submit it to Congress, then they’d have to pass a series of laws creating the state, and that takes time. Enough time that maybe an election cycle changes the party structure to prevent it. Worth noting that Puerto Ricans denied statehood at the ballot box in 1967, 1993, 1998 and 2012. The GAO in 2012 found statehood actually would hurt the island’s economy and set them back as they would have a higher tax burden.
DC won’t happen without a long court fight. DC is actually created in the constitution as a federally administered area, not controlled by a state. Since you like facts, here’s why. I’ll even quote the source:
“In 1783, a crowd of disbanded Revolutionary War soldiers angry about not having been paid gathered to protest outside the building where the Continental Congress was meeting in Philadelphia, Pennsylvania. The soldiers blocked the door and initially refused to allow the delegates to leave. Despite requests from the Congress, the Pennsylvania state government declined to call out its militia to deal with the unruly mob, and so Congress was forced to abruptly adjourn to New Jersey.”
After that incident, the founders realized having the nation’s seat of power in a state meant the state could control their access and/or deny them protection during the lawmaking process. To maintain impartiality, they decided, it needed to be an area without bias (I think all of this is in Federalist 43). So, the Constitution set out a plan to make that happen. Maryland and Virginia both gave land to create DC. VA ended up taking their contribution (modern day Alexandria) back after a while.
So it’s not an open and shut case for DC. It’ll be in court for forever.
These racist, fascist founders were real idiots, I tell you.