Are you allowed to go downstairs even if you are not buying a piano?

I think so

When I went down there last spring they told us we couldn't go down. Also there's only one upright piano there.

Yes you can do it, I went there with a sales rep

You can add the sellers, sometimes you can go there if you have some pianos lessons or if you practice. Maybe you can try by saying you would like to see the grand pianos ?