Assuming that Mew's attack and special attack are the same (maxed EVs) and both Poke we can see how much damage Mew will do with different EV spreads. For the purpose of this experiment, damage will be measured in percentage of total HP.
To start, let's do 252 HP, 128 defence, and 128 special defence. If Mew uses Focus Blast, it will do from 11.8% to 14.1% damage. This, times 2, will make Celebi take 23.6% to 28.2% damage.
Despite what you say, let's do 252 HP, 252 defence/special defence, and 4 defence/special defence. When Mew attacks, it will do 10.6% to 12.6% damage to the 252 EV stat, and 13.3 to 15.8% damage to the 8 EV stat. Combined, this makes Celebi take 23.9% to 28.4% damage, slightly higher than the previous option.
Next, let's try 252 defence, 252 special defence, and 4 HP. Each move Mew uses will do 12.5% to 14.9% damage. This, times 2, will make Celebi take 25% to 29.8% damage. This is much more than to previous two options.
The last test will be with 172 HP, 168 defence, and 168 special defence. Celebi will take 11.9% to 14.3% damage per attack, making it 23.8% to 28.6% damage for two attacks. This is slightly lower than the other 3 options.
As you can see, the option of 252 HP, 128 defence, and 128 special defence provides the best defence to this scenario. It is miniscule, with about 1 or 2 HP being saved.
Now for scenario two. Let's start with 128 defence and 128 special defence. For each attack, Celebi will take 14% to 16.7% damage. That's 28% to 33.4% damage for 2 attacks.
Next, let's do 252 defence/special defence and 4 defence/special defence. Celebi will take 12.6% to 14.9% damage in the stronger stat, and 15.8% to 18.7% damage. Combined, this is 28.4% to 33.6% damage, once again slightly more than the first option.
As you can see, doing 128 defence and 128 special defence is the best option, but only saves about 1 to 2 HP. I hope this all helps.
Site used: https://calc.pokemonshowdown.com