Regarding the link query ...
You started in a Sawtooth survey and I assume you have linked to another survey via a Terminate question (within the Sawtooth survey). At this point, you should have exited the Sawtooth survey as an incomplete status and inserted the url to where you need to skip to (the other survey/software).
When you pass fields/data (like an ID or password), the other software will have it's own way of handling that? This I can't help you with as it varies from one software package to the next.
The other software I assume has a feature that enables you to return to the Sawtooth survey? You need to use the same survey link that got you into the Sawtooth survey to begin with but include the additional feature "sys_skipto" which allows you to define the returning question in the Sawtooth survey. The example below shows the survey returning to Q11 in the tep1 survey.
You can check the Sawtooth help which has a solid explanation of "Composing Terminate/Link Questions".
Regarding the back/previous button ...
This is handled under the Survey Settings (the screw driver icon) under the General Format tab. You will see Navigation Button options. You can select Graphical or Text. For graphical, you select the path of the Submit and Previous buttons but for the Previous button you need to select the tick box to indicate you want it activated, otherwise it won't be displayed. For text, the same rules apply except you simply enter the text to display rather than selecting graphical files. Did you select the tick box because this will stop the previous button from displaying?
Good luck, hope this gets you sorted out.