Is the flex cable Ok? Are completely you sure you attached the flex cable correctly after mounting the new trackpad?

As Tommy Gurreri suggests, was the fan correctly connected after the cleanning? Other possibility is did you put too much thermal paste? Notice only a little is necessary. Using too much may be as harmful as using none, because the thermal paste is only needed to fill the microscopic gaps between surfaces, not to create a new layer by its own.

OK, so you swapped the HDD with a new formatted one. Assuming this drive is in perfect condition and correctly connected, The reason why after the chime (the sound) the folder with the question mark shows up is because the system does not find a bootable drive (I think EFI doesn’t have set up to boot from USB drives by default). So It’s most likely that you don’t boot from the USB drive or even that USB bootable copy was not correctly made. Try to start up with the drive attached and pressing and holding the Option (⌥) key immediately upon hearing the startup chime. Release the key when the StartupManager appears, showing you two possible options: your internal HDD and the install USB drive. Hope this helps!

In order to do a correct diagnosis, try to connect the laptop to an external screen. If you can see the Desktop correctly, the problem is the screen or the cable that connects it to the mainboard. If you continue to see nothing, then the issue is mainboard related.

Recently I refurbished a Macbook A1181 (version 4.1 , the one @2.4Ghz) and I had the same issue. What I did was sticking a tiny little piece of a plastic sheet between the metal case and a little gold V-shaped connector that is spotted in the left side of the battery connector board (if you watch it from the pins side), to avoid contact between them. This worked for me. Hope this helps.

Recently I refurbished a Macbook A1181 (version 4.1 , the one @2.4Ghz) and I had the same issue. When I finished the repair and mounted the keyboard and trackpad back on, as well as the battery, I realized they where not responsive, despite the power button still worked. I read in a forum there was an issue with this particular model and the ground level, making that both the keyboard and the trackpad don't work. What I did was sticking a tiny little piece of a plastic sheet between the metal case and a little gold V-shaped connector that is spotted in the left side of the battery connector board (if you watch it from the pins side), to avoid contact between them. This worked for me. Hope this helps.

Recently I refurbished a Macbook A1181 (version 4.1 , the one @2.4Ghz) and I had the same issue. When I finished the repair and mounted the keyboard and trackpad back on, as well as the battery, I realized they where not responsive, despite the power button still worked. I read in a forum there was an issue with this particular model and the ground level, making that both the keyboard and the trackpad don't work. What I did was sticking a tiny little piece of a plastic sheet between the metal case and a little gold V-shaped connector that is spotted in the left side of the battery connector board (if you watch it from the pins side), to avoid contact between them. This worked for me. Hope this helps.

Have you tried to reset SMC and NVRAM/PRAM?

Give us some more details, please, just to be clear. You have tried the SAME hdmi cable in ALL the hdmi connectors of your TV? First thing to do sholud be try with another cable. If the failure persists, it is clear the cable it's OK, so the problem is in the TV. Know check in every hdmi connector. With this action you can identify if the error is in a connector or if it is chipset related. If it is only one the non working connector, maybe redoing its solder points can fix the problem. If none of them works, it is a deeper problem. You can try to fix the board yourself if you have the skills and the tools or take it to a technician, or by a new board for your tv based on the PCB part number.

Normally the components you can upgrade in a laptop are the memory and the processor. The graphic card is integrated in the logic board. I don't know what you think it's an outdated computer, but in my opinion an i7 with 8gb RAM and a graphic card with dedicated memory is good enough for most uses, even videogames. Actually, the portable versions of a processor is less powerful than its desktop version as long as it is intended to be used in mobility, where the mos important thing is the autonomy, the energy consumption, so the designers prefer energy efficiency to power. Maybe that's the real reason why you feel a bad performance in an state of the art processor. TIP: have you cleaned up the laptop to remove dust and changed the thermal paste with a high performance one? TIP: you can upgrade RAM to 16Gb (2banks of 8Gb each, swapping the installed). In DDR3 it's affordable and it may work for you.