ESP8266 Flash - Maximize SPIFFS size
No.

Even if you don't use OTA, your program is stored in flash. If you have 1MB of flash, there's no way you can use all 1MB for SPIFFS; that would leave no space for your program.

Here's a page showing how flash is divided up in an Arduino environment.

Even if you eliminate space for OTA images and the EEPROM, the Arduino environment will still reserve space for the sketch itself and for its wifi config and possibly a little more data.

You can never use 100% of the flash storage for SPIFFS.