I don't have background other than reading the background included in the decision, but it seems that in late 1800s/early 1900s, the land was allotted into parcels with individual ownership by members of the Creek Nation, and after a time that ownership became freely conveyable to any person.

Conveying property to a non-member does not remove land from a reservation, and this ruling does not void any conveyances. Whoever owns the land, owns the land. This ruling just makes it clear that all of the land within the treaty boundaries is still a reservation. I don't know all of the implications this will have, but I imagine a lot of things done by the state of Oklahoma assuming they were operating within the state of Oklahoma may need to be reviewed, given that they were operating within the Creek Nation.

For example, can the state allow formation of counties and cities within the Creek Nation? What about if said counties and cities were formed inclusive of only land owned by non-members of the Creek Nation?

Of course, the court mentioned that Congress is free to stop honoring the treaty, the only requirement is that it must do it explicitly.

Here is Washington state, sometimes tribal land is leased to non-natives for 99 years. Will definitely be interesting to see what happens, but it is quite possible that "property tax" could be moved over into a land leasing fee.

This is how it works in Arizona as well. I think it's pretty standard.

Around Lewiston, Idaho, the Nez Perce tribe has been buying up land as it comes onto the market. They're also doing land swaps with non-tribal land owners for other reasons - better wildlife acreage for better farming acreage, for instance.

Land that is privately owned will not be affected. The tribes often manage large amounts of reservation land in trusts, but fee simple/patent land (land privately owned outright) is protected by the constitutions of the states and federal government. Reservations are managed by the federal government via the Bureau of Indian Affairs. However, the tribes can establish many zoning rules, but they can be successfully challenged by private owners.

Here is a recent significant case about this matter where the courts sided with the non-Indian private land owner regarding a permit he received from the county even though he refused to also go through the Reservation's own permit process: https://caselaw.findlaw.com/us-9th-circuit/1651530.html

I have no idea, but a policy what would make sense to me would be to respect existing private property, though layer a "right of first refusal" for the indigenous nation to buy it back if it comes up for sale.

Why should the rights of stolen property be respected?

If I buy a TV that "fell off the truck", I wouldn't expect the manufacturer to buy it back from me. I wouldn't expect the manufacturer to buy it back from my grandkids either.

In many places, good faith sales are respected. Say someone bought a TV that "fell off a truck", but then after a year sells it to another person insisting it was legitimately acquired. The second buyer could be a legitimate owner given the they purchased the TV in good faith. On a side note, even if you did legitimately purchase the TV why would you expect the manufacturer to buy it back from you after it was used?

Regarding the realities of the situation in Oklahoma: as of this ruling non-natives on native land outnumber natives more than 4:1. If the government aimed to take or redistribute private property I would expect major unrest as a consequence.

All land is stolen property, there are very few direct inheritance lines going back for more than a few hundred years. There are none that go back thousands.

To give an example from the german jurisdiction, if you buy a TV that fell off the truck, you are in posession of the TV but do not have ownership of it (however, you are not required to actually give the TV back, you can do what want with it).

If you sell that TV to a third party, ownership passes to that third party and the original owner looses all property rights on it. One of the few examples where ownership can pass from a party to another without the first party's consent (in neither express or requirement).

I imagine US law isn't totally different there.

It certainly makes sense for government/crown land to be given back to indigenous nations that hold title.

You could do the same thing I suggested albeit free for the Nation and the private landowner seller could be compensated by the government (which stole the land and resold it in the first place).

[Not US lawyer, but] an option open to some common law courts is leaving the legal owner in place while declaring that the thing owned is for the benefit of someone else (i.e. a beneficial owner).