The answer to (1) depends on what applications you're running.  If they're RAM-intensive, 12G beats 8. If you're not even using the full 8G, you'll get better performance out of 4+4 than 4+8.

If you want dual channel operation (the reason for the performance increase just mentioned), the modules need to be matched pairs. That takes care of questions 1-4.

If you're buying a mismatched module, look for a vendor willing to guarantee compatibility - and avoid both extremes:  the very cheap (ValueRAM, Value Select) and the high performance (Vengeance, HyperX, etc.).  None of these have good track records with Dell systems.

Thanks, @ejn63 
I use my system for coding(software which is heavy) & gaming(sometimes) so yes they are RAM intensive. Even without the software, RAM usage goes 80%+ with just Google Chrome.
So I will assume 8GB+4GB will be way better than 4GB+4GB RAM.

Can you please explain what "module needs to be matched pairs" mean? Same RAM size(4+4) or (Same company + Same part number) or (+Same bandwidth)?
P.S I don't have much idea about the dual-channel operation, I just read about it.
(FYI: I also have AMD Radeon graphics card on my system if that is related to anything)

Matched pairs means that for dual channel operation the modules need to be identical.

Thanks, @ejn63 
That's what my question was: What does module mean?

Same RAM size(4+4)? OR

(Same company + Same part number) OR

(+Same bandwidth)

Sorry, I'm not familiar with terminologies.
 

Each individual memory unit is a module.  To guarantee dual channel operation they must be identical (that word means exactly the same:  same manufacturer, part number, capacity).

Avoid the low end (Value RAM, Value Select) and the high end (Vengeance, HyperX);  neither has a good track record in Dell's systems.

Beyond that, whatever you settle on, verify compatibility with the manufacturer website.

Thanks, @ejn63  
I have done some research over the day and came to a decision to buy a different vendor's RAM(8 GB 2400 MHZ) as SK Hynix is not available here.

I searched on Amazon and found these:
-Kingston ValueRAM
-Samsung
-Crucial
-ADATA
-Corsair
-HyperX

Which one should I go for(it can be a different brand from above) that has good track records with Dell systems?
(Samsung one was quite expensive than the others)