  The upcoming James Webb Space Telescope (JWST) promises a generational shift
in the study of temperate mini-Neptune atmospheres using transit spectroscopy.
High-altitude clouds however threaten to impede their atmospheric
characterisation by muting spectral features. In this study, we systematically
investigate JWST instrument configurations for characterising cloudy
mini-Neptune atmospheres, assessing the importance of instrument choice and
wavelength coverage, focusing on NIRISS and NIRSpec. We consider two temperate
mini-Neptunes orbiting nearby M dwarfs, K2-18 b and TOI-732 c, with equilibrium
temperatures below 400 K, as case studies and assess observations using
different instrument configurations with one transit per instrument. We find
that their JWST transmission spectra with modest observing time and adequate
wavelength coverage can provide precise abundance constraints of key molecules
H2O, CH4, and NH3 even in the presence of clouds at significantly high
altitudes. The best constraints are obtained by combining all three
high-resolution NIRSpec gratings (G140H+G235H+G395H) that together span the
~1-5 $\mu$m range. Single-transit observations with this three-instrument
configuration allow precise abundance constraints for cloud-top pressures as
low as 3 mbar and 0.1 mbar for K2-18 b and TOI-732 c, respectively, assuming a
nominal 10x solar metallicity. The constraints vary with instrument
combinations. We find that NIRSpec G235H+G395H is the optimal two-instrument
configuration, while NIRISS or NIRSpec G235H is optimal for single-instrument
observations. Absent high-altitude clouds, even single-instrument observations
can provide good abundance constraints for these planets. Our findings
underscore the promise of JWST transmission spectroscopy for characterising
temperate mini-Neptunes orbiting nearby M dwarfs.
