As you noted, storing Historian data has been moved into the Historian module rather than coupled with the Store & Forward system (old HistoryManager, now called StoreAndForwardManager) for storage.
The most correct method would be to grab the Historian's StorageEngine using HistorianManager.get(gatewayContext).getStorageEngine(historianName), and then to store AtomicPoints through StorageEngine::storeAtomic. Creating atomic points can be done using DataPointFactory::createAtomicPoint.
You shouldn't need to use Reflection. New Historian SDK dependencies will be available on 7/29 that will expose historian-gateway and historian-common. Currently only historian-gateway is exposed through the historian artifact (which is now renamed to match).