I would completly change the soil.I'd wash off the root ball as much as possible and then stick it in some very hot water for about 10-15 minutes and clean& bleach out its container.and repot with some nice fresh potting soil.I had a bad case of spider mites in a AV plant once.I did think their 'cure' was nutz....but it worked.I also use Schultz garden safe 3 in 1 fungicide.I love it so much.....I buy the concetrated bottle to mix up.It will make 6 gallons it runs about $11 but can watch for it on clearance at end of season for about $6-8.They do sell a spray bottle already mixed for about $3.they sell a insecticide& then the 3 in 1 fungicide(which rocks.I just love to spray a bug& watch it suffer.)you can find it at walmart,but some only carry the insecticide.
your pics make my skin crawl.A spidermite can lay up to 200 eggs in a 2 week period.Under favorable conditions they will have a few egg laying partys& some of the 1st laid in a months time will lay some of their own& they'll form a comunity(yours looks like japan overpopulated).spider mite control takes 3-6 applications.The sprays don't kill the eggs(but the scalding hot water will)Cyflutrin would proably be better,but it cost $70 for 8 oz.it leaves a residue that keeps 'fighting' where the insecicide is shorter lasting.
Here's the product so know it looks like.http://www.epinions.com/Schultz_Garden_Safe_Fungicide_3
You'll have to treat weekly to get under control as when the eggs hatch...they'll be back.You can get it under control.My lil 4 inch AV pot was as bad as yours...yours is just larger.The hot water will get some eggs but not all.Neem is good also.but neem is added in the fungicide,but i've used both products& the neem you could use as a soil drench.I hope everything goes away real soon for ya.treat every 7 days till under control more....and treat the plants around it too.