1. No. Your maternal uncle's grandson has rights to till land. He might have some right to the property being a legal heir of your deceased uncle, but same can only be determined by way of filing a suit for partition in Civil Court. He cannot just encroach your land and start tilling without any prior consent of the court.
2.No. He has to approach court to get a favourable order against you,then he can take the possession of the portion as decided by the court.
3. Yes. Section 427 does apply.
4. This is a purely civil matter, but you still try to send your complaint under section 154 (3) CrPC to the senior officials of police describing your complaint and asking them to order investigation and registration of FIR in your case. You can also approach magistrate under section 156 (3) CrPC, if the senior officials also fail to register your FIR.
5. No. The chances are very meagre as he also has some rights in the said land.
Moreover, filing a suit for injunction against him with temporary relief should be done in this case by you.