SG
We assume CTR is a google ranking factor - as it just makes sense ie we know CTR is a ranking factor for Adwords so we assume it is a ranking factor for organic search results.
On lower case -v- upper case - It really depends on a case by case basis - there is no other way.
We do find that proper capitalization, english and a clean looking title have a positive impact on CTR. But it depends on each SERP page.
I will give you a few examples:-
Lets say your title was the one below
Thumbtack - Accomplish your personal projects
If the company changed to the below, they would increase their CTR for new customers. Note increased Capitalization and simply looks more professional.
Hire Experienced Professionals | Thumbtack
So this supports your hypothesis. However on the title below - Capitalization for each word has been used so based on the above as a general rule - it should have a better CTR. However it doesn't.
Cosmetics SFG Australia, Discount Make Up, Skin Care, H
Buy affordable makeup & beauty products | Cosmetics SFG
So by using the title starting "Buy affordable makeup.." - the CTR increased significantly. Why? To obtain this optimum Title, for SEO and clickability the company had to use lower case to come within the 512 pixels so the Ad did not truncate - and hence lose its customer friendliness.
I appreciate you are looking for rules but there are none. There are some general rules. ie at present from what we observe the pipe (|) has a higher CTR than the hyphen (-) - so we would recommend using the pipe. Also www. has a higher CTR than no www. on domain names.
But every instance needs to be carefully examined.
Finally say you are in the gambling space - our research shows - that when you want to gamble - it really does not matter what the Ad states - as long as it is passable, not off putting - it is all about position only. However for other SERP's ie "travel insurance" customers tend to scan the page in more detail. On those SERP's spend the time getting the balance between SEO & clickability right.
Not sure if I have assisted your argument to get capitalization, as indicated it all depends on your SERP that you are competing on.