I reckon your earlier post was right - start with fatter roll bars front and rear and see if that resolves the body roll issue. Uprated springs and dampers will probably ruin the ride.

Re the seats - I don't know the dealer etc, but I'd just query whether that's actually right or not. I would have thought the Octavia IIIs would all use the same airbag components. Would be interesting to see a parts list and compare the parts codes. If they're all the same parts, it should be a simple matter of disconnecting the battery, unplugging the cable connector under the seat, and swapping the seats out.

An easier (and possibly more cost-effective) solution would be to get the seats re-foamed and re-upholstered by someone that knows what they're doing. So long as there's no additional foam built into the side of the seat and the stitching is done so that the airbag can deploy (again, a decent upholsterer will know how to do this) you should be fine.

I'm a little nervous about the chip tune voiding the warranty. A lot of ECUs have flash counters built in these days....