Yeah I stand corrected

The incompatibility I could possibly think off buying a imported Xiaomi phone:
1. LTE frequency band (Band 3/7/38/40 are all supported on recently released Xiaomi's phone)
2. GPS (All Xiaomi phone support GLONASS, GPS and Beidou. Both GPS and GLONASS had global coverage and Beidou covered all parts of Asia)
3. Chinese hardware had issues running Global ROM (total bullshit spew by ignorant)
4. SIRIM certification are just making sure the electrical safety standard are met and MCMC are making sure the wireless transmission will not cause any unwanted interference in wireless spectrum.

For your information, the exact same Redmi 5 and Redmi 5 Plus just passed through SIRIM certification so I wonder could you care to educate us what kind of incompatibility that occurred with imported phones.

Because metal case act as a Faraday's cage. That's why there's a antenna line on phones with metallic body. It's also why wireless charging supported phone doesn't have metallic body.
You could read up more in the wikipedia
https://en.wikipedia.org/wiki/Faraday_cage

Is just the warranty are fulfilled by Xiaomi officially in Malaysia for Malaysia set. If the price difference are quite substantial, just go for import set since importer are doing a pretty good job at providing warranty service for phones they sold.

Redmi 5 Plus is better in terms of value considering the current price.

Sounds like your battery is worn out of charge cycle. Consider to replace it.