Agreed, I have not seen the type yet but M. rosenmuelleri seems to be a synonym of M. sutor, and was commented as this by Plavilstshikov (1958) who may have seen the type. M. s. rosenmuelleri has been recently used (and for iNat purposes) as the name for M. s. urussovii. See further comments from 2018 here: http://www.cerambycidae.net/news/06.08.2018/06.08.2018_europa_rem.html The original name was Cerambyx Urussovii, since urussovi has not been in prevailing usage it is an unjustified emendation and not an available name (see ICZN Article 33). I will explain why I believe it is a good species in the context of Monochamus in an upcoming paper.

I do not agree.
Considering Cederhjelm's orignal descriptions (https://www.biodiversitylibrary.org/item/44241#page/120/mode/1up) L. heinrothi was compared to L. sutor and. L. rosenmuelleri was compared to L. sartor.
It appears strange that Cederhjelm described twice the same species.
In fact, several differences corresponding to two species can be noted: elytra spotted (heinrothi) or uniform (rosenmuelleri), body size smaller (heinrothi) or larger (rosenmuelleri), elytra with ocellate (heinrothi) or strong (rosenmuelleri) punctures, legs grey pubescent (heinrothi) or black uniform (rosenmuelleri).
Accordingly, the provided pictures (https://www.biodiversitylibrary.org/item/44241#page/383/mode/1up) show two different species.
In addition, several other characters typical of these two species can be noted:
L rosenmuelleri looks grayish, with strong humeri and tapered elytra, while heinrothi is black with parallel-sided elytra.
In conclusion, the synonymy sutor = rosenmuelleri is not supported.