[Citroënbender]

You're spot on.

Cutting back would have been to me, the sensible choice, easy to deal with the reo cut ends as (or if) exposed. Cut to line and depth, then a couple of kerfing cuts and knock it up with a small Kango. Two hours with a competent labourer.

They messed around for ages (days) kind of hoping the issue would somehow go away. It's a sea of "paper builders" (academically qualified, not come up through the trades) subletting the work to monkeys with low care factor and no cross-trade skills.

Full weatherproofing under the sliding door thresholds either side of this nonsense, will be practically impossible.