(2) I am a member of another union; can I still I join the IWW?
Yes. However your other union may try to prohibit membership in the IWW, but there is no legal way they can do this.
Paid staff members of unions may also join the IWW as long as they do not have the power to unilaterally hire and/or fire other union staff members.
On the other hand, officers in other unions may not join the IWW, and members of the IWW may only become officers of other unions under certain circumstances, as outlined in the IWW Constitution, under Article II (Membership), Section 1(c):
No member of the Industrial Workers of the World shall be an officer of a trade or craft union or political party. Branches may allow IWW members to become officers of trade or craft unions as long as these exceptions are reported to the General Administration and no IWW member receives significant pay (more than dues rebate and expenses) as a result of being an officer or official in a union that does not call for abolition of the wage system.