Extension drainage - would you move pipes beyond and back into shared sewer?

[Duvetstealer24]

Ok - I'm quite stressed so please be gentle! Would love to hear from anyone with experience of this similar house.
We are doing a single storey rear extension on a 1920s mid-terrace. We knew we have a manhole over our own lateral drain and needed to move that to extend but it wasn't clear until digging began.
The extension also goes over a shared sewage pipe, we've spoken to the water company and don't need a build over agreement (but the builder will build a protective lintel).

In order to build the full width available (which is 5.5m including walls) we need to relocate the soil pipe and manhole. It would cross OVER the shared sewer, flow into a manhole, drop into a trough that flows back into the sewer. This also requires replacing part of the sewer with a Y-junction matching section of pipe (I have found a sewer specialist to do that part and have applied to the water company for permission, which we're awaiting).

Two pics below, drawn by me, not to scale exactly but they show the flow!

It feels like a complex system with several connections, and if something went wrong, it would be under our house. On the other hand, perhaps it's really common and will run perfectly well. And at least there would be the manhole/inspection chamber to access all pipes if anything blocks. (However, we wouldn't want to do that routinely!)

We have 2 tween/teens and my husband works from home, as I sometimes do, so have long needed an office- that was the main driver plus our old lean-to was falling down. The plan is for a new smallish kitchen and office in the extension and turn the current kitchen into a downstairs loo/utility space.

I am now thinking we could forfeit the 4x1m of space to the side where the manhole and drain is, and build a slightly smaller extension (but with a larger kitchen) plus the tiniest garden office available. This would be a lot simpler and we could press on with the works, but loses us that space which is makes the high cost less worthwhile.

Am I worrying unnecessarily? Has anyone got a similar layout and it's all fine / you've had problems? My husband thinks we should press on with Plan A and I am erring to the smaller version just to ease my stress!

TIA for your feedback.

Water runs downhill so as long as the fall is to building regs then it'll work. The curve before the inspection chamber is where things might get blocked, but access would be easy.

Alternatively you could (with water board permission) divert the shared sewer out into the patio so it isn't actually under your house so access would be easier if there were a problem. I've seen that on a few extension plans near me.

Just re-read and it is really odd that you say this:
"we've spoken to the water company and don't need a build over agreement (but the builder will build a protective lintel)."

I've never heard of a water board not having a build over agreement in these circumstances (but I've only personal experience of two).

Yes I know, believe me I have spoken to the water company a million times about it to clarify and they confirmed we dont' need a BOA. I wouldn't mind applying for one if we need it but after some confusion they say we don't.
Thanks for the other reply, much appreciated.