Blank(i) - keywords to look out for "since it added considerably to the price of paint" & the reversal "but".
indicating though not readily accepted at the start, but once it caught momentum. So we are looking for a word which can somewhat mean "not readily".

"slow to be" fits this description

Blank (ii) - there is ample evidence which suggest that Impressionist wanted to just get out their houses and get inspiration from the world.
So we are looking for a word to mean "make or aid" this escape. "abet" is the only one which makes sense

Blank (iii) - interesting one. What Impressionists was unprecedented, so there was a lot of criticism which led me to think a possible to think a possible guess would be authentic.
Lets look at the options:
"finished" - somewhat related to the guess, moreover words preceding the blank back it
"magnificent" - a general trap option type, if you plug in the options, it may seem to fit, but note the general context, we also need words/keywords to back it - which is not the case here
"improvised" - doesnt fit

I was confused between finished and improvised for the 3rd blank, since the para says that "it was practical to produce a finished oil painting on-site" and "take their inspiration directly from the world around them". Is there a way to clearly reject improvised for the 3rd blank ?

to take their inspiration directly from the world around them. For the first time in history, it was practical to produce a finished oil painting on-site, although art critics would long argue if Impressionist paintings were truly (iii) ____.

if you take inspiration from the outside world and then you paint, after that, the paint itself is weird and you need to finish on site, therefore, our questions is : is finished of the idea the paiters had was another?

Not improvised