Do not try starting with the CPU/GPU removed. I would start with making sure everything has in fact been put back in its proper place. However, the symptoms you mention sound possibly like a bad motherboard.

Thanks for your replies.
So i put back in the hard disk, one memory card and clipped the wifi cables back on, still doesn't work. Also i put a piece of plastic in between the two wires from the open piece of antenna. Strangely, the piece of uninsulated antenna cable is also uninsulated in the online disassembly guide pictures (although i found a piece of loose insulation inside the case that seems so belong to nowhere else). Odd thing, the whole cable, it has two wires in it, is insulated - but a short piece is lying open and the two wires are touching.
Well, i have no idea what is wrong here. The mainboard looks fine, the sockets as well (removed all thermal compound for now), all cables are connected. I'll have a closer look at the small cables, maybe one got broken during the various disassemblies. Some of them are running right underneath the case fan, which got really hot last summer, the plastic insulation here looks quite hard-baked, but nothing did melt. Or maybe something (or me) caused an electric discharge shock. Humm... One of those paper-thin wires that connects the power on and status LEDs with the mainboard looks a bit shattered too from dozens of times of putting it in and out again, however the power on button does work. It's just strange that the lights flash up every now and then, but nothing else happens.
The last things which are still not in their places are the dummy (SD) card and some of the screws. Screws are not essential electric components are they ;) It's a tedious work, but i'll put them back in as well.

I don't have a test meter, but i thought of borrowing one. Tried starting the laptop with yet another multi-adapter, battery pack only, and both. With battery+adapter, the battery charge light lights up. In any case, numlock and capslock and power-on leds and the power button light flash up shortly, hard drive doesn't spin up, display remains off. It's like the mainboard doesn't get any power - or shuts down itself. I let these unsuccessful power on/shuts down again cycles run for a couple of hours, still nothing.
One more thing, i also tried a bios reset by shorting a jumper with a screwdriver. But that was after the crash.
While typing, i'm googling for what happens if the bios battery is dead. Seems very unlikely, but i'll check out this one, too.

Thank you for your hints :)

If the bios battery is dead, you will be prompted to set the date and time etc...

Usually anyway.

It is sounding more and more like the main board is dead. May e worth getting it in to a shop, if you can find one that will do either a free or fixed price assessment.

Have you checked and double checked all connectors are in the correct place and are correctly seated? Also check that there are no loose screws or any bits of shrapnel in the case that could cause a short.