Backstabbr uses Google accounts for login, but you don't need a Gmail address to have a Google account. You can create a Google account associated with your existing email address here: https://accounts.google.com/SignUpWithoutGmail.
A Gamemaster is someone who is helping to keep a game run smoothly, but does not play in it directly. Gamemasters have the ability to pause & alter adjudication (say, if a player is going on vacation) and edit game settings. They can also invite replacement players if a player drops out. As a reward, Gamemasters are granted the ability to read all press, so you can watch your players betray each other behind the scenes. Please note that a Gamemaster CANNOT submit orders for a power or alter the state of the game board in any other way.
A Gamemaster is no longer required. When you are creating a game, you can select the option to start the game as a player, and your resulting game will not have a GM. As a result, these games cannot gain replacement players, change adjudication, alter settings, or do anything else that a Gamemaster can do. It will run on auto-pilot, basically.

It depends on whether your game is Public or Private. In a Private game we assume you already know all the other individuals and so the game creator retains most of the critical Gamemaster functions, namely the ability to pause or change adjudication, replace inactive players, etc. Game creators in a private game will not be able to see orders submitted or private press from other powers as a regular Gamemaster normally would.

If your game is Public then the game creator retains none of these powers as the risk of abuse is too high. If you play in a public game you create you will not be able to change any of the game settings or adjudication once you have done the initial setup.

Click on the Gamemaster button in the toolbar, and use the form at the bottom of the resulting modal to send an email to the replacement player in question. That email will contain a link that confirms their interest in becoming a replacement, and once they click on it you will see a new option appear in the Gamemaster modal that lets you select which power they will replace.
There is a configurable setting on the game creation page that lets game creators specify the length of the very first turn, and that setting defaults to one week. This is meant to give players additional time to carry out their initial contact and negotiation, which is typically heavier in Spring 1901. After that first adjudication, your game will adjudicate on the normal cadence.
This is almost certainly due to Fast Adjudication, which is what happens when all players click the "Resolve orders once all powers have issued orders" button. The math behind how we handle that scenario can be a little confusing. A game has an adjudication period of T. When a game Fast Adjudicates, we look at the time remaining before the next scheduled adjudication (R). If the R is less than T, we add T to R to set the next adjudication time. If R is greater than T, we leave R as it is. So for example, let's say that a game is set to adjudicate once per day. If the game adjudicates after 12 hours, the time remaining is 12 hours. That's less than 24, so we add 12 to 24 and the next adjudication will be scheduled for 36 hours out, which is larger than the normal 1 day period. Then let's say that Fast Adjudication happens again 6 hours later. The time remaining at that point will be 30 hours, which is larger than the regular adjudication period of 24 hours, so 30 hours will still be the time remaining until the following adjudication. Basically, we wanted to always err on the side of giving people ample time to respond to turns.
We've considered it but for a variety of reasons it isn't something we're interested in pursuing at this time. We appreciate the enthusiasm this project has generated in the community and the desire to pitch in and help out, but at the end of the day Backstabbr is our baby and we don't feel confident that opening up and managing source code forks and merges is the right way to achieve our goal of building the best possible core Diplomacy experience.
We are fully compliant with the Diplomacy Adjudicator Test Cases (DATC). Adjudication errors are extremely rare at this point; it is more common that the players have a misunderstanding of the rules. If you're curious about why something adjudicated the way it did, consider checking in with the Diplomacy subreddit.
The ability to submit invalid moves is sometimes frustrating or confusing to players but it is the result of a deliberate decision. Diplomacy is a game of deceit and being able to claim that you unintentionally submitted bad moves is one way to lie to another player you're trying to backstab. That strategy is used in in-person games as well. This is why we encourage players to use the sandbox to test out their moves in advance.
Civil disorder occurs when adjudication happens and a power has not submitted any orders. When a power is in civil disorder, all units controlled by that power issue Hold orders. If units must be disbanded while in civil disorder, they units furthest from a home supply center are disbanded first.
Sorry. Email us at feedback@backstabbr.com and we'll try to help out and/or suck less.