I am going to assume your questions are in good faith.
While I know nothing about this specific campaign, I am a member of the union they purport to be joining and have worked on a half-dozen new organizing campaigns. Local 13000 is a real local, and it would make sense for them to be joining it.
Most of your questions can be answered by reference to labor law and the union's internal "law" (i.e. the CWA constitution, sector and local bylaws), both of which are available online. Here is what we can say at a high level based on those resources (and in a few instances, my personal experience of the union).
#3 - Per CWA guidelines for locals engaged in new organizing, the leadership of the unit will be an organizing committee that represents each work group in the sought bargaining unit. The typical guideline is a committee of at least 20% of the sought unit. The CWA does not go public with new units without at least of 70% of the unit expressing interest in writing ("signing a union card").
#6 - The use of the term "renegotiating" is misleading. Collective bargaining agreements supersede individual employment contracts by definition. Typically, the CWA conducts an election of workers in the unit to choose a bargaining committee, which then bargains with the employer assisted by paid staff and/or lawyers. In any case, the contract must be ratified by a majority vote of the members for CWA to enter into it. Only then would their "employment contracts" be superseded. That said, the overwhelming employment contracts in the united states do not bind the employer in any meaningful way, and are not meaningfully negotiated. Where there are meaningfully bargained contracts, such as in broadcasting or entertainment, union members can and often do allow certain provisions to remain in force, using the union contract to set a floor on conditions. This is very common in Hollywood.
#7 - This depends a great deal on where Fastmail conducts its business in a legal sense, and is a somewhat unsettled area of law with increasing relevance due to remote work. In the United States, unions have a "duty of fair representation" to represent every member of their unit, have basic democratic processes, etc. This is typical in democratic countries. It is also typical for union members to equally share the cost of running the union to ensure that there is no free-riding. In the United States, federal law mandates that unions have a duty of fair representation, but some state laws allow people to avoid paying their share of the costs of that representation. These so-called "right to work laws" were proposed in response specifically to African American workers in the South seeking to form unions that could not be undermined by free-riding (cf. the Memphis Sanitation Strike).
#8 - The union may propose any unit. The employer may challenge that unit. If challenged, the National Labor Relations Board determines whether the workers in the unit represent a "community of interest." This standard evolves constantly as successive administrations appoint members of the National Labor Relations Board.
#9 - This is determined by the executive board of the local, which is elected by members according to procedures in the local bylaws. In some locals, this will be a fixed amount; in others, it will be a fraction of gross income. I pay 1.67% of my overall income in dues and saw a raise substantially greater than that when we ratified our first contract. Some members saw raises as high as 10%.
#10 - See the bylaws for meetings in the local. The unit meetings would generally be organized by the elected bargaining committee, and then by stewards. All locals of the CWA will have a business meeting monthly, but individual units (i.e. groups of workers at a given company) typically meet more often and delegate someone to go to meetings of bodies representing larger groups. Activity in unions is, generally speaking, highly uneven. In my unit, there is some sort of union discussion in our slack daily. Tech worker units are, as a rule, more active than those in other fields.
#11 - See the local bylaws, sector, and international bylaws, which lay out a comprehensive system of elected offices, delegation of democratic authority. The primary recourse is to petition their elected officers/stewards, many of whom will be their immediate peers and have an interest in maintaining good relationships. Failing that, they can support the candidacy of new officers in regular elections. Failing that, they can run themselves. Outside the elective process, all officers are bound by the decisions of collective bodies like the local's executive board (which are recorded in meeting minutes all members have the right to access). Major decisions about union policy go to a vote of delegates to the international convention (there was one last year), many of whom are rank and file members. If any officer (or member) violates the the bylaws, the CWA has procedures (laid out in the links below) that allow members to bring up officers up on charges. Charges are ultimately evaluated by juries with rank and file representation. Finally, under some circumstances there could be appeal through the courts under federal laws like the Labor Management Relations Act and Labor Management Reporting Act.
Local Bylaws: https://www.cwalocal13000.org/_files/ugd/5e3432_517e9a7e2c71...
CWA Constitution: https://cwa-union.org/for-locals/cwa-constitution