1. No, a right of way gives rise to no right to park. Essentially, a right of way is a species of easement. It only gives teh right to pass and repass over someone else's land. In order that there would be a right to park, there would have to be another species of easement known as a parking easement. Here, your neighbour has no right to park their car on the right of way.
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3. Dear *****, you should state something informally to them once you get the opportunity. It is best that they don't labour under any illusion that they can park there. So, when you get the chance, you should state it to them.