I went to make the repair and realized I have 1.5" pipe, not 2" (stupid). I was originally surprised at the amount of water accumulation, but I'm not well versed on how much condensate to expect. I have a cup underneath the drip right now, and collected maybe a few ounces in a 24 hour period. I can tell you this: After coming down from the roof, the line goes horizontal and from visual inspection appears to be pitched in the right direction. After maybe 8' of horizontal run there is a short 90 elbow, also horizontal. Then there is a short, say 1" section of pipe connected to this 90. This 1" section is mated up with a pvc coupling ring. This coupling is mated with about another 4" section of pipe until it meets a 90 oriented vertically. The drip is between the coupling and the 4" section of pipe. It appears they glued the wrong length pipe and then used a coupling to fix the problem. There must be a small gap between the coupling and the 4" section of pipe where the water finds a low spot. My plan was to cut the long 8' run of pipe just upstream of the horizontal 90, and also cut the 4" section of pipe after the leak. Then put new couplings on the existing pipe and connect a new horizontal 90 with new 1.5" pipe. I've done a number of pvc glue jobs and one thing that always bothered me: When I dry fit the pieces to check spacing, I can only twist the various parts together so much. When I apply glue the pieces then slide together tighter due to the lubrication, and then my spacing is off a little. I'm reluctant to push the pieces together less than the full amount when lubricated for fear of getting a leak. Do you oversize your piping sections a little to account for this, or just leave a little slack with plenty of glue? I haven't had any pvc leak on me yet, but always struggled with this. I thought maybe to glue everything together except the last coupling, and then dry fit that last connection and cut down the pipe as needed to make the final fit.

You have discovered a perhaps old and abandoned curb stop and riser box. If you were able to shut off water, I am guessing you have anouther valve at the street? What is leaking is the weep hole that allows the water service line to the house to drain back when this service valve is closed. Perhaps you can check to see if it is not fully opened which should close this opening. Sweating copper is never done on water services, although I know it could be done. Flared & compression couplings and unions are sold for this purpose and depending on your jurisdictions, dictate what they prefer. Compression is easy, if you have to flare, additional tools will be required.

Yes, an accessible backwater valve located on the main building drain is the most typical way to prevent this problem. It is a one-way 'flapper' attached to the drain/sewer line designed to allow water to flow to the sanitary sewer freely, but closes to prevent sewer water from backing up into the house from the sanitary/storm sewer in the street. Unless the water that flooded your basement/crawlspace came from water overflowing from storm grates in the street that rose to such a level that it flooded your yard and came in through vents or windows....then a backwater valve is the cheapest and simplest solution.

"Thoughts on best approach for re-work of siding/flashing greatly appreciated. Have heard that it might be best to cut into the brick and attach step flashing." Best way (my opinion) to attack the chimney/asphalt shingle area is with flashing and counter flashing. The flashing is typical step flashing. A piece of metal bent 90-degrees, one leg gets fastened to the roof deck with each course of shingles and the other leg runs vertically up the side of the chimney, floating (not secured) against the side of the chimney. The counter flashing covers the vertical leg of the flashing that is floating against the chimney. A groove is cut in the mortar joint and an edge of the counter flashing is inserted into that joint, held in with soft metal wedges, then the gap is sealed with sealant. The counter flashing is folded down to cover the flashing. It's a 2-piece flashing system where the two pieces move independently of one another. Here's a reference drawing to better show what I was trying to describe. When flashing a chimney, it's best to use a durable metal; typically copper or lead, or lead-coated copper. Something as durable as the chimney itself. If using aluminum, a thicker stock is more durable than the thinner gauges sold at most box stores. Your cricket or saddle? Due to the geometry I prefer to not shingle them, but they can be, especially if larger in size. Small ones are typically wood framed and completely covered with metal. Probably the most common slap-dash chimney flashing is simple single-piece step flashing that is "sealed" to the side of a chimney with several tubes of pookey. That is not the way to go.