As far as I can see in this example: How to prevent Graphviz from moving nodes to accomodate an edge - Help - Graphviz - splines=polyline; is an attribute of a graph as a whole: it says in splines | Graphviz that it is "Valid on: Graphs".
And, as far as I can see, there is nothing similar to splines in Edge Attributes | Graphviz.
So, quite probably, it is not possible to mix two types of edges (polyline and spline) in the same digraph -- but I'd still like to make finally sure: for instance, due to being "Valid on: Graphs" - would splines maybe be also valid on subgraphs? If so, then maybe one could use subgraphs, to group edges that should be styled as, say, polyline (or spline - btw, I tried this, did not work for me).
In any case, is there any way, to choose a subset of edges on a Graphviz dot diagram, and have it rendered with a different splines setting (say, polyline), from the setting of the parent/container digraph (say, spline)? (if not, an explicit answer would be much appreciated).
Yes, the splines attribute is a (Root) graph-level attribute. (all or nothing). There are no edge-level overrides. (though that would be nice).
However, there are several ways to combine multiple spline type, but not in one invocation of dot (or any of the layout engines) (see below)
The first question should be "Is this really necessary?"