I did outline the reasons in the description but
Basically I got the idea looking at some data on metropolitan opinion in the Algerian war before Evian, (I forgot the exact paper but you can find it on Persée.fr), it highlighted how most of the metropolitan population just didn’t care much about the pieds noir (not exactly surprising considering their treatment later). In this timeline, a junta has been leading France between 1958 and 1968, with 5 more years under a rigged very conservative government, despite ever increasing popular protest from large parts of society, after the election of Defferre, in the mind of most people Algeria is linked to the dictatorship, and beside because of the partition many metropolitan thought they could manage themselves alone, more cynically, the place and the War was a money sink and prevented further européenne integration, and Algeria had switched to the right decisively since the start of the war
At the same time by 1973, a MFA-style movement had appeared in the army since the war had been going on for nearly 20 years, after the socialist government was elected and they allowed he army to leave east Algeria, many metropolitan soldiers and generals were relieved and didn’t want to fight again, that alongside contradictory orders led to an indecisiveness at a time of increasing tensions, since many of the various settlers inhabitant of central Algeria (Altava), were scared of this prospect, and some extremely conservative part of the army had been sidelined by the government, -and finally the fact that many maronite and Lebanese in algeria had seen France abandon Lebanon to Syrian invasion, they took the destiny of the country into their own hand and declared independence from France, many didn’t expect it to last, but for reasons pointed above, the Defferre government accepted it.
The POD is rather early - the creation of a maronite state during ww2 - that allowed to tie France to the Middle East and an anti Arabs policy while opening a migration source for such Christian Lebanese/Syrian toward Algeria.
And yes there would be a lot of ethnic cleansing, the IRl Peyreffite plan for partition already planned for the deportation of 500,000 Arab Algerian , in this timeline since there is an actual war, many More would have to flee.