Can I extend my tourist visa on July 12 if it shows a used stamp?

You are confusing a couple things.

The VISA is valid for 90 days from the date you get it <- meaning you have that long to ENTER thailand. When you enter it's stamped used (as in you USED it to enter thailand ;) )

What lets you stay IN thailand is the entry stamp you got in your passport (not the one you show that's on the TM.6 departure card). There's a stamp just like it stamped IN your passport <- that is what let's you be here for the 60 days.

AND

you can extend that by 30 more days at the immigration office for 1900baht

It’s a single entry and you entered on it so it’s used.

You can still extend it by 30 days.