Arsenal looked ragged and this time Tadic's attacking intent paid off, Long darting beyond Hector Bellerin before beating a dithering Shkodran Mustafi at the front post as Petr Cech was left to berate his team-mate.
Stephens was shown a straight red by referee Andre Marriner for reacting to Wilshere's jersey pull, with Elneny quickly following the Southampton defender down the tunnel after raising his hands to Cedric.
Aubameyang's goals won't do much to salvage this sorry league campaign for Arsenal, but at least they offer a bit of hope for the future - as well as bailing out the deficiencies in defence.
Lacazette's cause at Arsenal has not been helped by the January arrival of £55m frontman Pierre-Emerick Aubameyang, but the 26-year-old believes he can work with the Gabon star rather than against him - with the pair having shared the goals during a 3-0 victory over Stoke in their last Premier League outing.
Yet Welbeck did not dwell on that bad miss and instead popped up at the back post to head home Alex Iwobi's cross in the 81st minute.
- Charlie Austin has scored in each of his four Premier League games against Arsenal (four goals), including in both of his appearances at the Emirates.
But Welbeck atoned for his miss before the eventful finale.
It all provided a dramatic finish to a game that had largely felt like a prelude to Thursday's trip to CSKA Moscow, with Arsene Wenger clearly keeping an eye on the second leg of their Europa League quarterfinal.
- Arsenal have won six matches in a row in all competitions for the first time since May 2017 (also six).
The 18-year-old forward Reiss Nelson made his first Premier League start for Arsenal and was given a rude welcome from Southampton midfielder Dusan Tadic, who was fortunate to escape a booking for his unsafe tackle in the opening minutes.
Javier Hernandez salvaged a priceless point for West Ham in their battle to avoid the drop after coming on as a substitute.
The visitors did not look like scoring until Chicharito came on with 20 minutes remaining.
"He proved himself at Dortmund, he proved himself in the Champions League".
There were tributes and a minute's applause for former Chelsea captain Ray Wilkins before kickoff.