Social Work will be required to carry out a kinship assessment of yourself and your partner and will make a decision whether to proceed with placing the child with you or proceeding with permanence.
The child will currently be on a supervision order through the children's hearing system and depending on the stage the panel may have given adoption advice to the sheriff. If the child is currently living with the prospective adopters it would be more likely that the adopters are petitioning the court directly for adoption.
The sheriff will set a court date for a preliminary hearing and will ask social work to provide an adoption report. Social Work have a legal duty to gain the views of the wider family in relation to the adoption and assess any potential kinship placements. If a suitable kinship placement is available then this should take preference over an adoptive placement. As you have said prospective adopters are only acting as foster carers at present.
However, the prospective adopters have a child legally placed by supervision order with them and they will be legally represented so could challenge any attempt to verify the supervision order through the children's hearing (and ultimately the sheriff court). Basically, it could get messy as it is at such a late stage but you'd have a strong legal argument. My advice would definitely be to seek legal advice from good family lawyers (criminal lawyers have started doing family law coz of legal aid changes and are often very poor at it).
If the child is placed for adoption you will have the opportunity to write letters and send photos at the start. It is unlikely they would consider direct contact if you have never previously had contact. Letterbox contact may be an option. This will be considered through adoption hearing in court- sometimes it will be written into the order, other times it will be a voluntary agreement with social work. Letterbox contact does not necessarily mean the child will receive a letter every year but rather will receive info at a time that is appropriate for them, when they are ready to learn about their biological family. You may receive a letter with updates one a year, sharing photos of the child may bit be considered safe if it is thought it could compromise the placement.
What is in the best interest of the at this stage is difficult to assess but legally a kinship placement is preferable to adoption. It is my view, however, that the local authority will likely do anything they can to prevent another move for the child at this late stage.
again- seek proper legal advice