What you should really do is hire counsel. Contacting a witness could be perceived as a crime itself (tampering with a witness), so I would suggest you do not attempt that. Your story sounds like you have made a few mistakes, you should start doing things right and the first step is to hire a competent criminal defense attorney. Good luck.
The age difference will not be a problem per se, however it will probably trigger a higher degree of scrutiny. If your marriage is for love and not just for immigration benefits, you have nothing to fear. However, do not do this on your own! Hire an attorney to make the process smoother.
Your half sister is still your sister, there is no discrimination for half siblings, so yes, you can petition for her if all the other requirements are met. The fact that your mother died when you were little does not change anything. However, waiting times for siblings are very long, so you should be patient to reunite with your sister.
I am assuming you are not Cuban yourself. If your Cuban husband obtained his GC as a CU6 then you should not have any problem, just send your I-485 (no I-864, as this is not a petition). However, recent BIA decisions may impact your ability to adjust status if he is not CU6. Consult an Immigration attorney as this is not a simple matter.
Unfortunately, you have to wait for your I360 to be approved to be able to adjust your status. You mentioned that you had an EAD that was expiring; you could request an extension of your EAD based on your original grounds. You should definitely consult with an immigration attorney.
Completing PTI does not fall into any of the questions you mentioned above. After completion of PTI the State will Nolle Prose the case, therefore there is no conviction, w/h of adjudication or suspended sentence. However, they will find your arrest and you will probably be questioned further.