1) The workitem type needs to have some attributes which have
multiplicity 1 to many. How do I specify these in the template and
also how to define the corresponding editor presentation ?

2) Is there any way I can group together some of the custom attributes
into a type and then define a single attribute in my work item type to
refer to this type. Again this attribute can have multiplicity 1 or
many.

We currently support only custom attributes of primitive types and
enumerations, all with a multiplicity of 1.

https://jazz.net/learn/LearnItem.jsp?href=content/docs/work-items-cust/index.html

There are two presentations (Multi-select list and checkbox) that may
help with the multiplicity 1 problem in the case of enumerations, however.

Expanding Work Item Customization -- among other things to overcome
these current restrictions -- is high on the priority list for 1.1.

3) The work item type can be linked to only specific type of other
work item types. How do I restrict what artifacts types a work item
can be linked to ?

You can restrict the link types themselves somewhat by using
permissions. But finer-grained control over the work item types on the
other side of the link is currently not possible to my knowledge.

Please open an enhancement request for this.

Regards,
Patrick
Jazz Work Item Team