I feel I should add a disclaimer that this will only answer the first question you posed on how the terror developed

The Reign of Terror was born out of increasing radicalisation of the revolutionaries. This has a number of key roots/causes:

The betrayal by Louis XVI

Out of the growing religious crisis, the National Assembly issues the Civil Constitution of the Clergy, 17 July 1790. The CCoC had a number of effects; abolished the tithe, freedom of religion was enshrined in Declaration of Rights of Man, nationalised Church land, dioceses replaced by the departments, priests paid by the state, Law of Residence (Bishops required to reside in diocese), election of priests and bishops. This is taken one step further on 27 November when the clergy were made to swear an Oath of Obedience to the CCoC, splitting priests into juring (those who swore) and refractory (those who did not). There were rumours that Louis XIV was taking sacrament from refractory priests, showing a clear disloyalty to revolutionary ideals. Contrary to the king's previous support of the revolution. Furthermore, on 21st June 1791, Louis XVI tried to flee the country only to be caught and captured in Varennes. Couple with this was his veto of a law that would exile refractory priests, particularly due to the high numbers of refractory priests in the Vendeé.

Legislative Assembly

Further radicalisation in the National Assembly was carrying out throughout this time. France's first constitution was issued on the 30th September 1791. 1st October saw the Legislative Assembly replace the NA, former members of the National Assembly were barred from sitting in the new LA and so younger, more radical politicians were elected. The new house (and the next one, the National Convention) was dominated by Girondins (Brissot) and Jacobins (Robespierre sat as part of The Mountain).

The Revolutionary Wars

Nearly everyone in the government in France wanted war in the 1790s. The King was in favour of war as he believed the revolutionaries would be defeated. The Girondins wanted war because they believed the country would be reunited. The revolutionaries believed the Duke of Brunswick was going to reinstate former emigrés on the outset on an planned invasion. Essentially they had misinterpreted the Declaration of Pilnitz, in that Prussia and Austria intended the declaration to be a deterrent and not a formal declaration of war.

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The Legislative Assembly went on to suspend the king, leading to a petition from the Jacobins to have him replaced by no one. This lead to the Champ de Mars Massacre on July 17 1791. As we know, the king was executed in January 1793 and France was declared a republic by the National Convention in September.

The Committee of Public Safety is established 6 April 1793 to protect from internal conspiracy. As such in May/June 1793 the Girondins were purged, in 1794 more political purges occurred removing the Herbertistes and Danton. A lot of the accusations at the time originated out the War of the Vendee and the Revolutionary Wars, anyone who was accused of helping the opposition was removed from power.

The start of 'The Great Terror' was the issuing of the Law 22 of Prarial (10 June 1794), the law simplified the process by which the Committee of Public Safety could try and prosecute, even more so than the Law of Suspects had already in 1793.

For me, the creation of the new legislative bodies were key to this. The previous members couldn't sit in the new houses and so radicalisation was perpetuated by having new, young, radical men less-experienced in national politics. This radicalisation was initially started by the king turning his back on the revolutionaries and the growing development of war.

Sources:

• The Oxford History of the French Revolution - William Doyle

• Interpreting the French Revolution - Francois Furet

• Lectures notes and conversations with Dr Leighton James, Swansea University

edit: apologies if not all details are filled in, I wrote this answer as procrastination for writing a uni assignment.

Edit3: /u/DonaldFDraper does a good job of answering your other two questions.

To add to his answer I would point you in the direction of the Paris Commune established 9 August 1792. The Paris Commune was established by the sections of Paris to form an alternative governing body to the central French government. The Commune again feeds into the radicalisation process I've talked of. On the 10 August 1793, the sans-culottes (the real driving force of radicalism in terms of militance) massacred the Swiss Guard and the Royal Family were imprisoned. The Paris Commune were known for spreading radical, revolutionary principles and even supported and endorsed the September Massacres in 1793.

The Revolution didn't really turn into the reign of terror but rather it's a part of the Revolution. However it was a result of the actions and attempt to stop the Revolution by Louis XVI. The famous Flight to Varnennes is a turning point of the Revolution. While Louis was vetoing many bills and acts, trying to stop the Revolution, it was too much and the evidence needed for the Revolution Assmebly to indict the King for treason.

The idictment does bring another problem, the idea of France Without a King. As a result, the problems of the Revolution start to appear. Without a king, moderate or royalist factions start to work against the Assembly in Paris, and the Revolution is portrayed as a Revolution of Paris rather than of France. As a result, there is a growing divide in France, something that will aggressively show up in the Vendee, a pro Royalist counter revolution that devolved into needless bloodshed and destruction from both sides.

With the Revolution being second guessed, there was great fear driven by the newspapers, most notably by Marat. This fear was channelled through the Committe of Public Safety whom collected supposed counter revolutionaries and royalists and had them executed. It was made worse when the Jacobins acted to clense the Revolution of moderate thought by killing or driving off the Girodians, the moderate Revolutionaries that were let by Danton.

However it would end with the death of Robespierre. The moderate forces of the Thermidorian Reaciton would have Maximilian Robespierre and his brother Augustin executed for their actions but generally pulled back the Terror in their counter cleanse of the Jacobins from the French government. Napoleon was also one of these that suffered imprisonment but was released within months.

The military had no real role of the Terror. The French military was actively trying to fight multiple wars on the German, Italian, and Spanish borders, trying to extend and export the Revolution. They were watched over by political commissioners for counter Revolutionary fear after the defection of Durmoiuz and other French generals to the opposite side.

Is this referring to the period of dissent and execution following the revolution?