How effective is a level A hazmat suit for immersion in deadly substances?
Note that I'm used to standard lab safety measures including gas masks and masks mith independent air supply but I never used a Hazmat level A suit in research. You might want to take the following with a grain of salt!


  
    Sodium cyanide 
  


($\ce{NaCN}$) is a solid with a melting point around 560 °C. While it is a solid, jumping into a pool of it shouldn't do any harm. Level A suits are like "your personal bubble" with independant air support. They are however not that heat resistant: Do not jump into a pool of molten sodium cyanide!


  
    High concentration (100%) Hydrogen Peroxide
  


3% solutions of hydrogen peroxide ($\ce{H2O2}$) in water are used for wound disinfection, solutions up to 18% are used to bleach hair. (In German, the term wasserstoffblond was used for bleach blonde).

Much higher concentrations or waterfree (100%) ($\ce{H2O2}$) is another league. 

Hydrogen peroxide can decompose into water and oxygen:

$$\ce{2 H2O2 -&gt;[\textrm{cat}] H2O + O2}$$

The reaction releases some heat, may happen spontaneously and is catalyzed by some metals. Add some burnable material that can be vapourized and you have a rocket propellant. Think Messserschmitt Me 163.

If the hazmat suit is tight, and the material withstands "bleaching" by the hydrogen peroxide there's still the question whether hooks, valves, visor frames or any other metal parts, that might be part of the whole gear will catalyze the decomposition.


  
    80% hydrochloric acid
  


That does not exist in water. Water won't take up that much hydrogen chloride gas under normal conditions. Concentrated hydrochlorid acid (~ 38%) should be fine.


  
    80% nitric acid
  


That's close to fuming nitric acid and again a pretty strong oxidant. I'd say that you're safe, but don't take my word for it ;)