Soviet Sniper, Vasily Zaytsev, and Enemy at the Gates, what was truth, what was fiction, and what was propaganda?

How historically accurate was the movie Enemy at the Gates?

It is not accurate.

Were the Russians that brutal toward their own troops?

Define "that brutal" for more precise answer. But if you mean "shooting their own en masse" then no.

how historically accurate are these texts

Vasiliy Zaytsev memoirs are about average in accuracy - some facts checks out, some does not. For example, famous sniper duel doesn't check out.

has propaganda influenced the journal?

Depends what you imply here. Let me guess:

Was journal written by political officers under his name? No.
Was journal edited or influenced by political officers at a time of writing? Doesn't appear so.
Was Vasiliy influenced by state propaganda? Of course he was.
Could Vasiliy hold off something that could be found inappropriate by political officer? Quite likely.
Did Vasiliy always kept in mind that journal could be read by somebody else and if something out of line is found, he could be punished? Most likely. Note that official policy in RKKA explicitly prohibited diaries at front lines - out of fear that captured diaries could expose troop movements or help german agents to infiltrate. Agents behind enemy lines were very active on both sides, in many cases russian POWs were recruited by Abwehr and sent across frontline to infiltrate troops and cause panic at appropriate moment. Nikolay Kuznetsov exploited diaries and personal records of captured german officers to pose as german officer, to spy and conduct several high-profile diversions for USSR side.

Did the Russian barrier troops fire upon retreating allies in a similar blood bath as portrayed in the film?

Absolutely not. There were few documented cases of barrier troops shooting at retreating soldiers and almost always they shot overhead, to stop panic. Every case of fire on friendly troops was investigated and unless found justified, commander of barrier troops would face severe punishment.

In most difficult cases it was enough to shoot one-two "obvious panic instigators" to stop disorganized retreat. And most of time, shooting over heads or at ground near feet was enough to stop panic. Later, panicked soldiers were "filtered" out (by NKVD and later - by SMERSH) and those found guilty of inducing panic could be subjected to military tribunal. Death penalty wasn't meted out that often. Most often punishment was assignment to penal battalion (=штрафной батальон=штрафбат) or penal company (=штрафная рота). Note that both barrier troops and penal troops weren't unique to RKKA - Wehrmacht had those too.

Reports for Stalingrad Battle specifically:

For Stalingrad battle - from 140755 stopped and checked:

3980 arrested
1189 shot (either on spot or by firing squad after military tribunal)
2776 sent to penal companies
185 sent to penal battalions
131094 returned back to their units or reassigned to another unit

Sources

(russian, online) Translation of unter-offizier Aloise Heighmesser(?) diary. Showcases keen interest of NKVD to personal diaries of captured germans. Translation is addressed to Abakumov, who was high-ranking NKVD officer in 1942 and in 1943 was assigned to lead SMERSH.
(russian, online) Collection of NKVD documents regarding Stalingrad Battle. In many documents you could see that barrier troops were fighting Wehrmacht, acting as a last line of defense, sustaining heavy casualties (up to 70%). Quite often proven veterans, survivors of 1941 were assigned to barrier troops and that explains why a barrier company would held off the line after their regiment (full of conscripts) ran away. Some division commanders were reprimanded for using barrier troops as front-line troops without any pretenses.
(russian, online) State security organs of USSR in Great Patriotic War. Collection of documents, vol 3. = Органы государственной безопасности СССР в Великой Отечественной войне. Сборник документов, том 3. Volume 3 includes documents about Stalingrad Battle.