Why didn’t Jesus use the word for adultery in Matthew 5 & 19.

Jesus said that divorce was allowed because of the hardness of their hearts. What exactly was the sin involved. Were the men putting their wives away legally or illegally before the allowance was made or were their hearts hard because the interpreted the allowance as being able to divorce for any reason. What was the hardness?

How do we get the narrow meaning of adultery out of a general word like porneia?

Is there any way that we can defend that Jesus’ instruction only applies to the betrothal period?

In 1 Corinthians7:10 He tells the husbands and wives not to depart but if one does depart the departing one is told to not remarry or to be reconciled. What about the remaining spouse? Can they remarry?

What is the word and its meaning for being under bondage in vs 15? Is it the same as being bound or loosed in vs 27?

The Church is not currently married to Christ but is betrothed to Him?