that is at the discretion of the immigration officer on the day. But you shouldn’t have a problem doing this once, but you may well have a problem trying it a third time
you don’t have to do a border run to get the additional 30 days, you get that extension in any immigration office. After that you have to leave the country & re enter for another 60 days exemption but that is at the discretion of the immigration officer you get on the day