I have a contingency in place for the soil test to ensure there is no contamination, but as it's fully operational does it make sense to immediately remove the tank and install an alternative heating source? 

They'll probably try to deny the claim as "contamination" which isn't usually covered.

or contaminating it with methamphetamine etc Re Survey: Is it something that you would regard as must do?

Remediation for any issues on a Phase I can quickly escalate upwards of $100k if there is contamination in the ground soil on the property.

Environmental contamination from old tanks is a huge concern on an old gas station like that.

You'll also likely need to do a Phase I Environmental (again in the $2500 range) which pulls up the history of chemical spills, soil contamination, water quality, etc. 

Also the water is tested for lead and other contaminates.

Is there an EPA filing for possible contamination?

Consider an environmental study due to contaminated waste and chemicals used in the medical field.

We told them they had to remove their buried oil tank to go forward and we would reimburse  them at closing if there were no contamination.