Good points! As a retired facilities engineer, I still have contacts in the mechanical industry and am currently in contact with my former prime plumbing vendor, now retired, seeking a referral.I would encourage you to seek a second and/or third opinion if you haven't come to the same conclusion already. To go right to wanting to hire an excavator to expose the full run kind of feels more like they want to jack up the bill. It might be the right thing to do ultimately but if I were you I would only be inclined to go along with that if the had at leadt tried cutting out the damage area and replaced with new clear pipe. If they did that first and it didn't fix the problem it would strongly suggest there were problems in other parts of the line as well. Only then would exposing the whole line would make sense to me. To often these days it feels like mort repair peopl just want to make the bill as big as possible with not regard for whether the customer can really afford such a large bill. They seem to operate under the premise that since you couldn't do it and had to hire someone, you'll pay almost any price if what they are fixing is important enough to you..
Casse in point, recently my well pump died on me. I'm not sure how old it was, I just know it was the one already in use here when I bought the place 23 years ago. The plumber came and verified the pump was old and shot and needed to be replace then he started talking about other switches and valves and such that needed to be replaced as well and he quote a price oof $1400 for parts and then additional labor which he wouldn't price in advance because he wasn't sure how long it would take. I thanked him for his time, paid the obligatory $100 for him to even come look and give me a quote (although the $100 would be deducted from the price if I hired him). I had a 2nd plumber come look and he came to the same conclusion but he quoted a price of $1200 for parts with labor included. His quote didn't include any seperate line items for valves, switches, etc. I told him another plumber had looked before him and indicated that the other valves, switches, etc. would also need to be replaced. He kind of grinned and said that was true but then went on to say that when you replace the whole pump those things come included in the package. I hired him and had running water again about 30 minutes later (not counting the time it took him to go to the plumbing supply he uses to purchase the pump, but he didn't charge me for that). One of my better experiences with 2nd opinions.
I dug down to the broken pipe, leaving open the question of how far back the cast iron pipe is busted up after the 120-year-old Douglas Fir's root ball it was passing through, was uprooted in a windstorm.
What I would like for them to do first, is run a camera with beacon through the cleanout and identify where the breaks start, then using their above ground locator to establish where the camera is under the ground. That would eliminate any guessing.
I can already see we will lose our Rosemary bush, as well as our Mountain Laurel, and hope the break is after our 97-year-old Rhododendron and that we don't have to dig through our front cobble stone walkway.