You should have received the activation code in the email you provided when you purchased Sencha Complete. If not (or if you lost it), please write an email to firstname.lastname@example.org to obtain the code.
No, if you buy Architect you will also need to purchase a license of Ext JS for every user of Architect. Another option would to buy Sencha Complete.
No. For example, a 1 Developer Pack Sencha Complete bundle can be used by only one unique developer.