A new state of 'Columbia' would not be in the jurisdiction of another state and would not be formed by the junction of two or more states.
You do comprehend that DC currently was formed by the junction of two States, correct? If the national capitol is not going to be within an existing State, and you want to abolish Article I, Section 8, Clause 15 that created DC in the first place, then that only leaves US territories to place the national capitol.
Are Democrats seriously considering moving the national capitol to some US territory, like Guam or the US Virgin Islands? Because they can't leave it in DC if they make it a State. The Twenty-Third Amendment must also be repealed by another amendment before DC can be made into a State. Which requires the approval of 38 State legislatures. So that is a foregone conclusion that it will never happen.
DC would remain the federal capitol. It would be smaller. Article I, Section 8, Clause 15 has zero to do with this and I'm not catching your point about Article IV, Section 3, Clause 1 of the US Constitution. See section 2 of the 23rd. Congress shall make laws to enforce the Amendment.
It cannot be both a State and the national capitol. It is either one or the other, but it cannot be both.
Article I, Section 8, Clause 15 is what created DC as the national capitol. So if you intend to make DC into a State and not alter Article I, Section 8, Clause 15, then you must find a new location for the national capitol. Like it says, a location where Congress has jurisdiction "over such District (not exceeding ten miles square) as may, by cession of particular states, and the acceptance of Congress, become the seat of the government of the United States..." That includes moving the White House, Congress, the Supreme Court, and all the monuments to the new capitol location.
Article IV, Section 3, Clause 1 of the US Constitution requires not just congressional approval, but also the approval of any State legislature that shares the border of a newly created State. When DC was chosen as the capitol both Maryland's and Virginia's State legislature gave their approval to cede their land to form the national capitol. Approval from State legislatures would not be necessary to make Puerto Rico a State since no other State borders the island. The same thing was also true for Hawaii and Alaska when they were added as States. However, to make DC into a new State it would require the approval of both Maryland and Virginia State legislatures.
States are not awarded seats based on total population, they are awarded on a formula that includes population. Why does WY (@ 580 k) have the same number of reps as ND (@760 k)?
States are awarded seats based on total population. It is the percentage of the total population of the State in comparison to the total population of the rest of the nation. Wyoming (578,759) has 0.175% of the US population (331,449,281). South Dakota (884,659) has 0.267% of the US population. Which is why both States only have one US House Representative.
Based on the 2020 Census, it requires a minimum population of 1,000,000 or 0.302% of the total population before a State can be awarded their second US House Representative seat. Which is why Rhode Island (1,059,361), Montana (1,068,778), Maine (1,344,212), New Hampshire (1,359,711), and Idaho (1,787,065) will all have two US House Representatives and why Delaware (973,764) still has only one US House Representative.