Advise:

Wa alaikum as salaam wa rahmatullaahi wa barakatuhu Sister,

To start with the easiest of the issues, it is prohibited to masturbate. That is supported by the command to be patient, and to increase in acts of 'ibadah, and to fast nawafil. 
Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen:

Question: What is the ruling regarding masturbation?

Response: Onanism is masturbation with the hand – which is prohibited; It is obligatory upon all Muslims to take precaution against it, because doing it is contrary to the statement of Allaah (‘Azza wa Jall):

{And those who guard their chastity (i.e. private parts, from illegal sexual acts). Except from their wives or (the slaves) that their right hands possess, – for then, they are free from blame; But whoever seeks beyond that, then those are the transgressors}, [Soorah al-Mu.minoon, Aayahs 5-7]

In addition to that which exists in masturbation of much [health-related] harm.

And Allaah is the Expounder of All Success.

In regards to your other questions:
Determine whether you want to stay or divorce.  That's the first decision YOU need to make. There is nothing you can do about him being in prison. Unless he did not commit the crime. In this scenario, I will be presuming that he did commit the crime.

Being constantly separated can be due to noble things as well. So first you have get past his separation not being for a noble thing such as work. To do this FIRST seek refuge in Allah. Allah truly is sufficient for all of your needs. He may supply you with a group of sisters that have gone through or are going  through this same trial. Maybe you all can form a support group. However, you must stay in constant prayer. So that Allah can lead your thoughts and decisions. Not others. 

People don't have to understand.  A marriage is between Allah, you and your husband. The ummah should support marriages, not impose their deal breakers on you. Allah hates divorce. But he also permits it. Permitting it doesn't mean you have to exercise that option. According to Sharia you can divorce if no intercourse for 2 months or more or if your husband does not provide you with what you need for living, such as food and shelter. In some cases, it is possible for an incarcerated husband to fulfill this yet the wife would still run full speed ahead to the divorce option. BUT YOU DO NOT HAVE TO. That's a decision for you to make. Not the ummah. And it seems you have made it. Now work on it. Write often, phone calls, social media etc All things he has access to. When people start to give an unsolicited opinion politely tell them their advice is not wanted or needed. And change the subject. 

Will you be rewarded....
Allah rewards us all for our good deeds. Yours can be many. Know that you won't be punished for exercising your right to divorce. Make your decision with a clear mind.

You don't need anyone's permission to stay. If that's what you want to do.