As I follow every Robert's move (I'm admin member of Kubica's Fans Club - robertkubica-klubkibicow.pl) I would like to explain few things about him.
Firstly his hand and his disability - few years ago Robert said he could pilot F1 car at 100% on most tracks but the problem was to turn steering wheel in very tight corners like in Monaco, so there were about 3-4 tracks he would not be 100% efficient. And he is totally ambitious guy so he refused to test F1 car in the past because he saw no point to do this when he was not ready to became fully capable F1 driver.
The main problem with his right arm was low wrist mobility, so he had to compensate it with movement of the whole arm. So tight cockpits was a problem, he felt comfortable in rally, GT and WEC cars as he had enough space to operate.
But all this time he worked very hard to improve this and as we see now he is successful, in recent test he operated paddles with left hand to shift up and down (as Herbert mentioned on Sky Jacques Villeneuve did it in similar way). When RK realized his F1 comeback dream could happen, he started to work hard on fitness in general and now he says he is in best shape ever.
Than in 2016 and 2017 he did thousands kilometers in simulators of Renault and Dallara, he completed 2 tests in GP3 car, and also in Formula E, choosing demanding circuits with tight hairpins.
And finally he completed full GP weekend simulation in one day in 2012 Renault/Lotus E20 car. He did better laptimes than Siriotkin - Renault third driver, he did consistent 115 laps, he could do even better laptimes, but there were no more soft tire sets (and the tires were new spec to him). The team, the engineers, were little bit shocked with his performance. Robert himself said he is proud of what he achieved and this statement is most meaningful for me, as he is very modest and would never overestimate himself.
I'm not native English so forgive me if I wrote sth. in wrong, or too complicated way, but I hope its all clear.
To sum it up, in my opinion Robert is truly ready to return to F1, whether this will happen or not depends on many factors (as everything else in F1) and as, before his terrible accident, he was considered future WDC contender, I would love to see him in a competitive team fighting for it in 2-3 years from now.