You could have a few things driving the cost:-Location - if the house is more remote the contractor will have higher costs to get out there (gas, lodging, limited resources) or if you are using a local guy he might be higher because he knows there are fewer competitors -El Nino - the roofers we use are all busy because people are rushing to get work done before the rains hit.  

There is also one big issue with your tax projection - the taxes of $280 are based on the vacant land taxes and will likely go up to ~$2000-$3000 in a year or so when the assessor catches up with you.I realize there is a 1031 involved so a real estate purchase is necessary, but you should compare your ROI to a safe yielding instrument like a 10 year T-note or AAA corporate bond yield. 

If you want to know for yourself, pick one of those address and drive around the area so you can understand why. 

Which is the best, relatively safe Philly neighborhood for finding a potentially cash-flowing small MF (3 to 4 units)?

True off market requires work by driving, street view, calls, mailers or other ways of finding true off market. 

Tenants are looking for parcels that fit in their ideal box size layout for building and parking spaces requirements with any drive thru's wanted.

Is vital oxide safe to use around the house undiluted?

There are other ways to do it - driving for dollars, snipe signs, mailers, etc.  

However if it is rough and deteriorating/cracking then you might want to budget a high amount for removing it safely (which is not cheap) then residing the entire property.I'd try to negotiate the price down either way!

Are they really safe?