Those temperatures are quite normal for a stock 8700K running on air cooling.
The chip will throttle at 100C and that's the temp limit (TJmax).
Also, 90C during a stress test (Prime95, IBT, Realbench, OCCT, etc) is fine.
Delid it if you're looking for 10-15C improvement in temps and you don't mind losing the warranty.

On your second question, I also think that the 4-5C might be caused by inadequate airflow. Like your suggestion, you may try swap with a better fan to see if got any improvement. Might as well upgrade your cooling solution to a better air cooler or AIO cooler.

For some reason, offset didn't work for me. I've been using fixed voltage all the time. vCore will fluctuate on load/idle according to Load-line Calibration (LLC) in your BIOS settings.

Noctua NH-D15S of course, world's best air cooler.
But make sure that it can fit inside your case because it's really huge. 

Upgrade to chromax heatsink cover and NF-A15 HS-PWM Chromax fans:



https://www.lelong.com.my/noctua-heatsink-c...7-01-Sale-I.htm

https://www.lelong.com.my/noctua-casing-fan...7-01-Sale-I.htm

or might as well consider upgrading to AIO cooler like the EVGA CLC280 or NZXT X62

No ala-carte.
FYI, out of the box the NH-D15S comes with one single NF-A15 PWM fan.
The cheaper option is to buy a used unit.
Resell the original beige/brown fan as it is valuable for someone who needs it. 

Yes, those are another set of MOSFETs/VRM. Not being cooled by any heatsink probably because they are a little weak (low-end mobo) and won't generate too much heat.

Moreover, usually board manufacturer would expect users of such entry-level would only use air cooler hence the cool air coming from the cooling fans on top of VRM area.

However, if you feel that this area is toasty then you may consider putting a 3rd party heatsink over there. You can remove the heatsink before sending for RMA.

Thermal pads are easily removable, won't stick to the MOSFETs.

I don't think TMPIN3 represents the MOSFETs temp. Usually named as VRM temp in monitoring software such as HWiNFO64. But take note that not all mobo provides VRM temp monitoring, only those higher end boards do. Yours is low end, so...