The MCP3008 CS is on HIGH
This statement does not make sense. CS pin of MCP3008 is an input. It cannot be "on HIGH". It can only be at whatever level you connect it to.
The GPIO15 works as HSPI_CS pin only in HSPI mode, i.e. with ESP8266 as slave.
In your schematics ESP8266 is a master. You can use pretty much any pin to control MCP3008 CS. Normally GPIO0 is used for this.
If your board has external flash chip then SPI is used for that. In this case you can try using HSPI interface. Connect GPIO15 to GND with 10k pull-down resistor. This will ensure correct boot. After boot you can initialize and use HSPI in master mode.