Aizen does not insert the Hōgyoku into Rukia's body. The deed was actually done by Urahara Kisuke when he makes a deal with a powerless Rukia for a human gigai into which he places the Hōgyoku for safety and secrecy.
Rukia by coincidence happens to patrol Karakura town on the day the hollow attacks Kurosaki family, the Hōgyoku belonging to Kisuke is hidden in the gigai which he gives after she transfers her powers to Ichigo; until then she can sense reiatsu and it was because of sensing Ichigo's reiatsu that she initially entered the Kurosaki residence as a shinigami. However after using the (Kisuke provided) Hōgyoku contained gigai to stay in human world, she realizes that her shinigami powers are not recovering fast enough. Aizen merely
Aizen likely orchestrates the hollow attack on Kurosaki family likely believing this would cause Kisuke, who until then hidden in the shadows would make a move for the safekeeping of the Hōgyoku. And rightly Kisuke makes an appearance offering Rukia a deal on the gigai to use for the stay in human world.
Also to be noted that the entire birth of Ichigo is a causality of Aizen's earlier experimentation and he mentions that he has been monitoring Ichigo for a long time
As for the extraction, Aizen merely assumes/contemplates that it would be of fatal consequence and expresses surprise when that is not the case. In general Aizen's knowledge on Hōgyoku and even that of Kisuke is not complete as they only succeed in creating an incomplete one at best both of which Aizen would later merge and use.