Hi Tim,
I can't speak to the digital signatures specifically, as we haven't implemented that here, but I may be able to point you in the right direction with some of the peripheral questions you asked, as we do use ADS for interactive forms over SSL.
The Java stack is the host for the ADS functionality. You can use any existing Java stack in your landscape, such as your Enterprise Portal, for instance, or you can install ADS on a dedicated Java stack (which is what we do, and what I recommend). So, the basic level of SSL communication is regular Java stack configuration, not specific to ADS.
The end users do not normally need their own userID on the ADS server. Instead, you will create a user called ADSUSER as a technical user on the Java stack, and then you will use the client certificate of your ABAP system (i.e., the SSL client SSL Client (Standard) entry in STRUST). Export the ABAP system's SSL client cert and import it to the Java stack's ICM SSL view, and as a client certificate for the ADSUSER. This way, your ABAP system can talk and authenticate to the ADS system over SSL as ADSUSER.
You will also need to create a system user on the ABAP system called ADS_AGENT and give it the SAP_BC_FPADS_ICF role. Then you will create a Destination on the Java system called FP_ICF_DATA_<ABAP_SID> that points to your ABAP system and authenticates with the ADS_AGENT credentials.
You will probably need to install some sort of Adobe credential to the ADS configuration on the Java system. For instance, for interactive forms, there is a credential called ReaderRights to be installed. You apply to SAP for this credential, and it is specific to your customer number. With this credential installed, your end users can use the basic Adobe Reader application to fill in interactive forms, or sign documents, or whatever the credential authorizes. They shouldn't need the full Acrobat product. The Adobe Reader will be called within a frame when they are logged into the ABAP system.
Anyway, that's a high-level overview of the basic landscape for SSL on ADS and integration within an ABAP scenario. I know it doesn't answer your digital signature questions, but hopefully points you in the right direction.
Cheers,
Matt