Force SMC reset? maybe?

If the battery symbol shows an X, you have likely shorted the pins when disconnecting the battery (usually with a metal tool), which will send battery voltage to the SMC. This will kill this section of the SMC, preventing it from recognising the battery. The SMC will need to be desoldered, reballed and soldered to your board, pulling it from a board of the same model and year as yours. Replacing the logic board is also an option.

With the power button issue, you have either damaged the keyboard cable when disconnecting it (either slightly ripped/poked a hole in it, or bent the corners of the cable), not connected it properly, or this is related to the SMC also.