  Penalty methods have proven to be particularly effective for achieving the
required $C^1$-continuity in the context of multi-patch isogeometric
Kirchhoff-Love shells. Due to their conceptual simplicity, these algorithms are
readily applicable to the displacement and rotational coupling of trimmed,
non-conforming surfaces. However, the accuracy of the resulting solution
depends heavily on the choice of penalty parameters. Furthermore, the selection
of these coefficients is generally problem-dependent and is based on a
heuristic approach. Moreover, developing a penalty-like procedure that avoids
interface locking while retaining optimal accuracy is still an open question.
This work focuses on these challenges. In particular, we devise a penalty-like
strategy based on the $L^2$-projection of displacement and rotational coupling
terms onto a degree-reduced spline space defined on the corresponding
interface. Additionally, the penalty factors are completely defined by the
problem setup and are constructed to ensure optimality of the method. To
demonstrate this, we asses the performance of the proposed numerical framework
on a series of non-trimmed and trimmed multi-patch benchmarks discretized by
non-conforming meshes. We systematically observe a significant gain of accuracy
per degree-of-freedom and no interface locking phenomena compared to other
penalty-like approaches. Lastly, we perform a static shell analysis of a
complex engineering structure, namely the blade of a wind turbine.
