1. Do hours built in the US fully count towards the 45 needed for an EASA PPL?
If you mean hours built other than on an approved EASA PPL course - No. You must have completed 45 hours of flight instruction (including supervised solo) at an EASA ATO.
2. If I understand correctly, in order to get a valid EASA license, I need to take the practical test in Europe, get a Class 2 EASA Medical and do the EASA ground exams.
Partly. You need a Class 2 medical and you need to pass the theoretical knowledge examinations. However, you have to complete an approved course, which may be done outside the EU and the skill test does not have to be taken in EU airspace. As BEagle pointed out, the approved course will include a mandatory 100 hours of theoretical knowledge instruction.
3. How different is the FAA syllabus compared to EASA?
Some, but not much.
Will I need to get a significant amount of re-orientation lessons back here after having done most of my training in the US?
If you have done an EASA approved course in the US, not much. If all of your training has been for an FAA PPL, rather more.
Your plan is not just bad, it's entirely impractical (apart from point 1.) - you must complete an EASA approved course, as part of which you must complete the required theoretical knowledge instruction and the training provider must recommend you for the exams.