1927 Main House Trap Clogged and Inaccessible

Help. I have a 1923 house that I just purchased with a 4″ Cast Iron Main Drain. This house sat empty for 2 years. (Old Foreclosure) After 1 week my main drain line backed up. I called a professional (20+ years experience) and he cleared out the drain. When he snaked it; a vent pipe inside the corner of the crawl space (non-accessible) of the home which exits outside through the foundation wall out came a tennis ball and other crap through the vent. (Does this mean the “U-shaped house trap is definitely inside the home in the non-accessible crawl space ?). But he hit a point (38 feet)where he could not snake it anymore where he thinks he is hitting the main house trap and he recommends that I find it, clean it out manually, and/or then replace it with regular pipe or I will be calling him back again in a month to 6 months. He thinks there is a bunch of rust or other sediment sitting in the house trap that will cause a problem again. Finding it may mean digging up the driveway or cutting a hole in the foundation wall or cutting a hole in the floor on the dining room above to access the crawl space. ( $$$) There are 2 cleanouts in different locations outside the home that he did not see when he was there but he thinks that may not matter because it will be after the house trap that is hidden under the non-accessible crawl space or outside. What is the best path forward ??:
1) How can I tell if the House trap is inside the non-accessible crawl space or outside based on the vent pipe inside the home exiting through the foundation wall outside? No cleanouts for it that I can see. Maybe the plumber is wrong on if there is a house trap and he hit something else or the snake could not get around the corner.
2) Don’t bother doing anything and wait and see if it clogs again. Maybe the tennis ball was the only problem or should I be proactive as I am waiting for another disaster backup to occur.
3) Hire someone and just try snaking it backwards from the cleanouts outside if the cleanout allows. Assuming the cleanout is only in the wrong direction here; not back towards the home.
4) Video it to verify it there actually is a house trap and to verify it is close to being clogged again and find its location (Can the video camera examine and pass through the main “U-shaped_ house trap ?). Is this a gimmick for a contractor to tell me then the line is a disaster and he has to rip it all out ?
5) Try High Pressure High Velocity Hydro Jet cleaning. (Does it have enough power to clean out a “U-shaped house Trap” and keep going or is this pretty wimpy. Can it pass through the “U-shaped house trap or will it get stuck or not clean it out enough.
6) Do the old fashioned way and spend $$$ to just start digging/demo until you find it and get rid of it. $$$
Sorry this is so long but any help would be appreciated. I am leaning towards video camera then high pressure hydro cleaning. If that does not work; Dig it up.

Let first say that I am not a professional plumber, so verify my comments before starting to work. That said, I am quite sure that main house traps are no longer employed. Each fixture in the home needs its own vented P trap. You can try the hydro jetting, I can't see it would hurt anything, but it is my opinion that the house trap should go and P traps installed as needed.

IMO, When a house sits unused for a long period of time, the "crust" inside the drain pipes dries up and flakes off in chunks. If there is enough of this build-up combined with old CI pipe, the potential for clogs is great. If the guy put a full-sized cutter on a rodding machine through your main line and achieved full flow, the drain is probably now clean. If he just ran a tape or a spear-head through it, it probably did not accomplish much of anything.

If you have a house trap it can be a problem as it can fill with solids. Hydro jetting works, but nothing will go through a house trap full of solid scale debris.

House traps have nothing to do with the traps on the individual fixtures. It has a different purpose.
1. If you have two closely spaced cleanouts one MAY be backwards to go into the house, and the other would go away from the house, which is the CORRECT way not backwards.
2. Almost no cleaning device will go through a house trap, neither will a video cable, but it will tell you where it is.
3. There is no way to tell where a house trap is located, unless you have access to its two vent lines, or the cleanouts on either side of it. These cleanouts would only be for the trap, NOT the pipes going to and from it.
4. Blindly digging to try to find it would be the absolute worse option, because your house could look like a bombed out building in Iraq and you might still not locate it.