Many thanks for your feedback!
About your first point, you should use the official syntax, which is a -l-> b
It's only by chance that the parser accepts a lazzy form for State Diagram. We could raise now an error message in that case, but we are reluctant to break any existing and working diagrams, so we won't. However, you should not use it anymore.
About point 2, you are right and this has been fixed in last beta http://beta.plantuml.net/plantuml.jar
This kind of inconsistency of syntax can be quite confusing. Is there a valid technical reason for it?
Sorry about that: the technical reason is that there are two different parsers for StateDiagram and DeploymentDiagram. This is because features are not exactly the same for both diagrams.This give a great flexibility for parsers but it means that we have to manually ensure consistence between both syntaxes, which is not 100% complete, as you have found.