It has been explained and covered many time by many individuals, you just have to go and find it. I will take some time to try to help your understanding, for the benefit of everyone trying to understand.
So, we can use any ammonium salt because when the lye breaks it down, the ammonia gas is liberated. The pressure condenses it back into liquid so that it will go to work reducing the lithium.
The EP/PSE HCl extracted from your pills (you may not even need to extract some pills) will turn into freebase in site because of the ammonia and lye used in the reaction. The EP/PSE MUST be in freebase form in order to undergo reduction from the solvated lithium. Ammonia specifically, because it will reduce and create a vehicle for the liberated electron to find the EP/PSE for reduction. This is where the agitation "shaking" part comes into play.
The ammonia strips the lone electron from the lithium, turning it a bronze/copper color in the process as it dissolves, this electron is attracted to the pseudo (through whatever molecular bonding mechanism - go open an encyclopedia on van der waals forcce or whatever maybe.) This all takes place in our non polar hydrocarbon - such as ether, naptha, etc.
There are by products and side reactions that happen - the important thing is that the only AMINE in the reaction is the EP/PSE being reduced into meth. It doesn't turn blue unless it is saturated with lithium, I guess.
There are things to avoid, things not to do. Water is a big NO-NO. Except for the initial 5ml to start the reaction, there should be absolutely no water involved.
Ether can be used, I believe it can be done without pressure as you mentioned, It doesn't give you any sign that it is finished or anything I don't think and then the worry becomes if the product has become 'over reduced' but I am just speculating, I have not experimented with any no pressure techniques.
This is why SnB will always have a special place in my heart. It is probably the most open reaction as far as what you can use and substitute, but also it follows some very strict guidelines and is very unforgiving. I'm actually somewhat fond of my SnB dope, it has a very potent 'initial rush' quality to it that I haven't really noticed from other meth I have tried.
This is just a short overview, but I hope it helps to explain some of the mechanics.
You shouldn't have much trouble finding an ammonium salt to use for this. Check ALL the pharmacies or shops in your area, if one store doesn't have it, doesn't mean none of them will. You just have to keep checking.
When you try to use the ammonium sulfate fertilizer, you can do a "dry-run" to test out if the lithium is going to bronze properly before you waste the EP/PSE in the reaction, if it does not work out right. I have not used it myself, but this will be my next option when my cold packs run out.
Good luck