Whether by update or bug, there is no way for me to select the game speed to be 2.0 upon the start of a campaign level.

William Wallace 1 is fixed at 1.5

El Cid 1 is fixed to 1.7

Under "game" the speed is set to fast but there is no button to select a speed.

The button becomes available in single player and multiplayer, but not in campaign. In single and multiplayer it will remember the speed I set it to last and start at 2.0.

The ONLY workaround is by using the hotkeys to increase speed, but it will always reset to either 1.5 or 1.7 at the start and I will have to manually shift it up at the start of each and every attempt.

Am I okay to proceed under these circumstances?

I can't give you any advice other than spamming the button just after closing the objective. Spamming it during levels loading will always put you in 2.0 however, except for WW where the game will mostly stop you at 1.7. And the only exception to use 1.7 is The Art of War. If you are trying to run any level outside WW/AoW, the game won't put you back to 1.7 if you're restarting it. So you might want to just enter the level, use the speed up key to set to 2.0 and then restart the level instead of quitting and re-enter, then start your attempt from that point on.