Tons of towing experience but not in a Mark VIII.
But a couple thoughts...
You want at least 10% of the trailer weight on the tongue weight. If you noted no propensity to 'fishtail' before, you'll likely be OK but I agree the air suspension has the ability to mask a lot of trailer issues just due to the self leveling feature.
Coil springs will need to be properly sized for much over 100 pounds tongue weight. If I remember correctly, the equivalent weight over the axle is 2-3 times the tongue weight in a typical bumper tow arrangement. So if you have say 150 pounds tongue weight, you can approximate it by putting 300 to 450 pounds in the trunk. Odds are 200 pounds or so would not be an issue, but 400?
If it were me, since the rear airbags are available and reasonably priced, I would keep the air ride in the back if for no other reason than the self leveling feature. But you don't necessarily even know what failed just yet?