A lot of this will depend on the specific device and version of the Android OS that the device is running. Newer devices and/or versions of Android support full device encryption, or encrypted app storage etc.
This means that any information that someone might be able to recover from such a device would be effectively useless to them without the appropriate decryption keys etc.
2. If the above statement is true (could be), the possibility of recovering an encrypted data that have been deleted from a device is 1?
There are numerous (unknown) factors at play here (device, android OS version, potential vulnerabilities, attackers resources like budget/time/skill/equipment etc), so it's really impossible to quantify the possibility with any certainty... so while the possibility may not be 0, it's not necessarily 1 either.
3. Is it recommendable to make use of the “single address account" that Mycelium supports, knowing that somebody can get hold of my device (either by buying or stealing) and recover the deleted private keys using some recovery Apps.?
I think the usual caveat for "mobile device" wallet applies here... don't use mobiles devices to store/use amounts of crypto worth more than the cash you'd be willing to keep in a physical wallet in your pocket etc.
...just need to clarify (To know) if they is a loophole that can be sealed or have a reason to avoid or proceed with the “Mycelium single wallet account" for people that use them as the best way to secure large amount of bitcoin.
And given this particular use-case involves dealing with a "large amount of bitcoin"... I'd be inclined to say "No"... it's not suitable. Unless the use-case is simply
spending the coins from an existing private key that was created offline etc... ie. import the private key, send ALL the coins (and any change sent to new address created offline etc), consider the old private key "compromised" and never use that old key/address ever again.
Even then, I'd only use Mycelium as a last resort or in an emergency...