The CROSS CONTAMINATION is really bad.

If other businesses have contaminents that have leaked into your soil that could be a huge development problem.You might want a developer consultant who knows that market to discuss further.I know nothing of Washington.

I haven't seen evidence of mold anywhere else in the property.I would like to fix this myself and avoid using remediation people if at all possible.1) Is the best course of action just to cut out those parts of the drywall/board that have it, so I don't risk contaminating other areas, rather than trying to scrub it off?

What I did not tell you , the loan term is amortized over 2 years, the park is only worth 450k and best part is since you were too hung up on a down payment, you did not dish out for a phase 1 and bought a contaminated property.  

A pawn shop takes all kinds of TVs, electronics and small appliances that, if damaged and left to sit can pose contamination issues.

To me 24 feet seems shallow but I would just insure the water is not subject to contamination from other sources (not to close to septic etc) and has adequate flow.

The purpose of containment is to ensure that airborne molds, dust, fibers, etc. do not get into the un-contaminated portions of the home.)Will you scrub the air?

Most developers will look at the remediation work that is needed, which happens when the soil has been contaminated.

Do not spend another dime on this property until that question is answered.It could be something as simple as an outstanding fine for an overgrown lawn or as disastrous as environmental contamination leaching into a public water supply.  

Not necessarily mean that property is contaminated.