Practically, it is possible to neatly undercut doors (the bottom edge would need to be resealed), if the work is done cleanly. Assuming standard door sizes are used (e.g., door dimension simplified 90 x 200 cm, corresponds to construction dimension 88.5 x 201 cm), this means that if doors are cut down, should the doors ever need to be replaced, special sizes will have to be used again, which are significantly more expensive than standard sizes. Depending on the design, doors can quickly cost 1.5 times as much.
The costs for moving the door lintels are not easy to specify, depending on the construction status (already painted?) and wall surface (plastered / wallpaper?). If it is still shell construction or no plaster has been applied yet, I recommend insisting quickly and decisively on moving the lintels. The stated deviations are significantly higher than the permissible tolerances in building construction.
More decisive is the difference in effort, whether the doors are in drywall partitions or in solid walls; in load-bearing walls, it is even more complex. If the door lintels in solid walls are only to be trimmed, I recommend consulting an external specialist. The critical point then is that the concrete cover of the reinforcement remains ensured – depending on the construction method. Cutting back the lintels up to 4 cm can lead to the load-bearing capacity of the lintels or the concrete cover of the reinforcement being jeopardized.
I also recommend checking the windowsill heights. It is important that the minimum heights of the sills (fall heights) are not undershot (heights are specified differently depending on the situation and federal state). Overall, check: where is the error, so that when installing the stairs, there is not another nasty surprise.
Conclusion: I would insist on remedial work, especially if the doors are 2 m high (with greater heights, this is at least tolerable in use).