To me the most plausible solution to form km-size objects from dust/sand/stones/snowflakes is via gravitational pull of a sufficiently large and sufficiently dense cloud of such bodies with very low relative velocities. Part of the cloud will collapse, part of it will be ejected.
At low temperatures near absolute zero ice/snow won't adhere much different from silicates.
In the
outermost part of the solar system (up to 1
lightyear resp. 63,000 au from the sun) relative velocities of objects, which remain in the solar system, should be below a range of between 100 m/s and 1 km/s, depending on the distance to the sun (otherwise particles
escape).
Collisions / friction should reduce relative velocities further until the precondition for
gravitational collapsing clouds is fulfilled.
Collisions of sufficiently large and fast bodies may lead to densified fragments, besides fine debris.
The
accretion process will take quite a bit longer than
Wiles needed to show the
Taniyama–Shimura–Weil conjecture.
The Sedna orbit may be explainable e.g. by an instable
ternary system of Kuiper objects, which eventually split into a binary system ejecting Sedna near Sedna's perihelion. I don't see the necessity for a big planet or star to explain the orbit.