10 month old baby congested from common cold. Won't sleep on flat surface n no swing. Is it safe to use carseat for 1 night next to couch?

It does have risks. The academy of pediatrics recommends a flat surface to prevent sudden infant death syndrome (sids). A more important aspect of helping the child sleep would be nasal saline drops and suction 2 or 3 times per day, but not more frequently than this. Certainly children can sleep in a car seat for prolonged periods without issue, but there is a higher chance of sids which must be considered.