Is it permissible to eat meat labeled "halal" in Germany, knowing that slaughter is prohibited there, and is the consumer or the seller held accountable if it turns out to be unlawful (haram)?

If the shop owners are Muslims, then their word regarding the meat being ritually slaughtered is acceptable, and mere doubt should not be entertained unless there is a valid reason for it. The default rule for slaughtered animals is prohibition until their permissibility is known through ritual slaughter by a Muslim or a person of the Book. Accordingly, the inquirer's ruling depends on the validity of the reason for their doubt; if there is a valid reason, then inquiry and investigation are obligatory, otherwise, it should not be considered.