compression test will tell you, first dry test, then pour some engine oil through plug hole and retest, if it goes up to normal figures, it is the rings, if compression stays bad, its the valves.
Iirc you were running without a t stat. I think your car might need a ring and head job.
Na na, t-stat valve was never removed and it is working alright with fan on auto.
I always used coolant, previously used Caltex one and since 2 years using Denso one.
Compression test was done back in 2012, don't remember the values but it came out to be alright for all three cyl.
I was also thinking about getting the compression tested again, but was unsure till now.