I will need to work through your blocks logic for a while, but straight off the bat, a problem will/must arise with your delay blocks
you should not introduce a delay in this way, use a clock timer to break up the processing.
I would also suggest that if you are sending 70 - 100 messages that you call down all the data from the google sheet in one go (which is happening but you have included blocks to call everytime??), process all the messages in the app, then send back all the data to the sheet in one go...and process each row with the web app.
Still not really sure why sent messages are not showing up in the messaging app for some users?