Its hard being a buyer!! It happens more often than you think. Just because you tendered an offer it does not mean a seller will negotiate or is even obligated to respond. Sound like someone bested your offer by terms and/or price.
There is no obligation to put any additional signage on the sign, but there is an obligation to report it properly to the MLS. It can either go under agreement or be kept active but showing for back up.
A offer is not accepted until it is fully executed meaning all parties have signed and are aware of the contract be executed. There is no set time.
To my knowledge ... there isn't a "binding" backup offer ... but there are professional understandings which does have risk and depends on your belief/faith/trust in the professionals involved.
Agents are required to present all offers to their clients. Do you have a reason to think your offer was not presented?
You should demand to see the Seller(s) WRITTEN and SIGNED response to your offer.