You have to maintain the status and can check your tier progress, basically to maintain Plat status, you need to achieve >3500 points by 31st Dec. It's calculated every 6 months. 

Could my tier be ‘downgraded’ a few levels down?
This would depend on how often you use Grab.

Here’s another example. Let’s say, your current tier is Platinum. You will stay on Platinum from 1 Jan- 30 June (Reward Period 1). However, if you take 0 rides during that Reward Period, your Platinum tier would be ‘downgraded’ to the basic ‘Member’ tier. That’s why you need to take more rides to earn a certain amount of points to stay on your current tier.

https://www.grab.com/my/grabrewards/

I don't understand your question. 

No such thing as unlock 1 more tier to stay permanent Plat. I have been Plat user since very long time ago and every 6 months, I still have to go through the 3500 points benchmark.

yes

Yes, fastest way is by spending at merchants as that gives 20x points. These days it's easy retain Plat status unlike last time I had to take a lot of rides just to maintain Plat status.