Any spacers that were on the axle would not have simply fallen off. I suspect you just need to honk on your axle nuts some more. Alternatively, if it looks like the dropouts are wider than the hub locknuts, you could pull off the locknuts on both sides and space things out evenly to take up that extra room. But that's going to be a pain in the ass.

If you re-tightened the axle nut with a multi-tool, you almost certainly did not have enough leverage to get it tight enough. You need a real (presumably 15mm) wrench to do the job right.

I'm having trouble imagining that the serrated washer would be distinct from your axle nuts. Usually, the nuts have a wide flange that's serrated itself, and no washers to loose.

In any case, you need things tighter.