AN A319 FO ENTERED AN 0.10 ALT CHANGE IN THE FMGC AS THE CAPT WAS ENGAGING MANAGED SPD ON AN IMC APCH. THE A/P DISCONNECTED RESULTING IN AN UNEXPECTED AND CONFUSING SERIES OF SPD AND ALT DEVIATION EVENTS.

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RNAV Y APCH TO ZZZ. CONFIGURED; ON PROFILE; ON AUTOPLT; INSIDE FAF. WE WERE IMC AND ZZZ WX WAS RPTED NEAR MINIMUMS FOR THIS APCH. ON INITIAL CONTACT WITH TWR WE RECEIVED A NEW ALTIMETER SETTING WHICH WAS 0.10 INCHES LOWER THAN WHAT HAD BEEN RPTED ON ATIS. THE AIRBUS USES THE MANUALLY ENTERED ALTIMETER SETTING TO COMPUTE A GS-LIKE DSCNT PATH TO MINIMUMS. CONSIDERING THE WX; I DECIDED TO ENTER THE NEW ALTIMETER SETTING IN THE FMS. WHAT NEITHER I NOR THE CAPT (NOR; FROM SUBSEQUENT CONVERSATIONS; 2 OF OUR CHK AIRMEN) KNEW WAS THAT CHANGING THE ALTIMETER SETTING WOULD CAUSE THE AUTOPLT TO DISCONNECT. I ALSO DIDN'T KNOW THAT AT THE MOMENT I WAS ENTERING THE NEW ALTIMETER; THE CAPT WAS PRESSING THE BUTTON TO GIVE SELECTION AND CTL OF AIRSPD TO THE FMS. AFTER AUTOPLT DISCONNECT; THE CAPT TRIED TO RE-ENGAGE THE AUTOPLT BUT WAS UNABLE. HE CALLED FOR A GAR AND SET CLB THRUST. TWR CLRED US DIRECT TO THE VOR AND 3000 FT. WE INITIATED THE PROCESS OF RECONFIGURING FOR THE GAR. ABOUT THIS TIME WE GOT A LOW SPD WARNING. CAPT MOVED THE THRUST LEVERS TO TOGA. TWR CALLED WITH HDG CHANGES; FIRST 230 DEGS; THEN 270 DEGS. IT WAS A BUSY; NOISY AND CONFUSING TIME. WITH TOGA THRUST WE BEGAN A RAPID CLB. CAPT THEN REALIZED THAT THE FMS WAS NOT CTLING THE AIRSPD; THAT THE FLT DIRECTORS WERE COMMANDING A PITCH ATTITUDE TO YIELD 155 KTS; THE LAST SELECTED SPD ON APCH. HE MANUALLY SELECTED A SPD OF APPROX 210 KTS AND BEGAN TO LEVEL THE ACFT. HOWEVER; BEFORE CTL WAS FULLY REGAINED WE HAD CLBED TO NEARLY 4000 FT. THIS CHAIN OF EVENTS STARTED WITH AN UNEXPECTED AUTOPLT DISCONNECT AND INABILITY TO RE-ENGAGE. THE DISTR THIS CAUSED DIVERTED US FROM NOTICING THAT MANAGED SPD; WHICH HAD BEEN SIMULTANEOUSLY SELECTED; HAD NOT ENGAGED. TEACHING THAT ALTIMETER ENTRIES IN THE FMGC DURING AN RNAV APCH WILL CAUSE A DISCONNECT WOULD HELP. GETTING A CLRNC TO THE ZZZ VOR STARTING THE MISSED APCH TOOK ME OUT OF THE LOOP AT A CRITICAL TIME. THE VOR ISN'T PART OF THE APCH SO I WAS TYPING INSTEAD OF MONITORING. USING EXISTING ELEMENTS OF THE PUBLISHED MISSED APCH PROC OR HDGS WOULD REDUCE WORKLOAD IN MODERN ACFT. LASTLY; I'D LIKE TO SEE MORE SIMULATOR TRAINING ADDRESSING PROBS RELATED TO CREW INTERFACE WITH AUTOMATION.