Might be S.O.L. On that one unless you can find a dead board that's identical to your board and are really good at soldering and unsoldering. You can try ghetto rigging it some how to stay down just be sure using stuff that is non conductive or anti static. And make sure you unplug battery while you are ghetto rigging it.
Usually that loading bar means running disk check or trying to recover from a power save deep sleep. Charge battery for a while then do smc reset and pram reset then Try holding option key at boot and try going to recovery partition run disk check and repair disk permissions see if that work if still doesn't work try using installation disk again sometimes when booting to CDs it takes a ridiculous amount of time seen it take as long as half hour just to boot to a dang cd so be patient and wait up to 45 min to be safe if still doesn't work. If have another Mac you can try to boot with the other macs hd in yours and vise versa. To verify if can boot to either drive. Then if not try to get your hands on an apple hardware test cd for your model of laptop test hd and ram
Ya if power button doesn't work then will have to replace top case for sure i do apple repairs and very common after liquid spills to take out just power button or will have to short every time to turn on
If you are able to get green light on adapter you can try smc reset left shift control option and power at same time might fix it make sure the MagSafe on the mobo is fully seated and make sure seated properly to side of case to get full contact with adapter also unplug battery try seeing if can power on with just adapter if nothing works might be motherboard issue or faulty battery indicator on side of machine. Check underside of motherboard for liquid damage or anything noticeably wrong if nothing find new board online or ask Apple store to do exchange with apple for new board