Pre trial diversion is considered an admission by the USCIS but is not necesarily a deportable offense by the court - depending on your current status. You need to consult with an experienced immigration attorney who can get the relevant facts from you and explain your standing before the USCIS and the immigration court.
The best situation for immigration purposes is "nolle prosequi" or charges being dropped. For the other portions of your questions, I agree with my colleague.
In any case, shoplifting, by itself, is not a deportable offense.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
I think your questions are very direct and require very concise answers. For question number one, retain an immigration lawyer to research the statute of conviction and the pre-trial diversion statute to determine if it has been previously found to be a conviction. The answer to that question drives the answer to whether or not there is a deportation, exclusion or other immigration consequence. Not all shoplifting cases are the same. An issue that does arise is the value the stolen property and its effect on your sentence. Pay an experienced lawyer to lay it out for you. Its better you know now than later so you can take corrective action.