If bitcoin is splitted after August,and someone send the btc to some other btc address during this period, will those in trasit btc lost for ever?
No. You can broadcast the transaction on both of the chain and when either of the chain dies off, the transactions will still be there. However...
If the coin were to be sent during this period, the transaction is also susceptible to replay attack which both of the coins in both of the chains will be spent to the same address. This could result in the loss of money if both fork were to live on.