My key is dropping to bits. - 2006 Mitsubishi Grandis 2.0

I'd use a gage and check proper fuel pressure, then with key off, see if the pressure holds for a bit, it should. If it drops like a rock have to check for fuel pressure leak.
Also, your evap system may have a leak? gas tank fumes.

If your vech. is fuel injected, check your cold start valve. It sprays a bit of fuel when its cold to assist. In starting.

Go up from underneath ,drop the radiator support cross member then let the rads drop down a bit then you can get a small 10mm spanner and put your arm to undo the two nuts on the back of the lock on the bonnet,i have done quite a few this way as the wand thing as its called here in spain packs up ,they are not cheap either if you can get the earlier one from a scrappers which is a telescopic rod then this is better

run fault codes and check for MAP/MAF/VAF sensors and the HO2S sensor operation

It might work if you disconnect your battery for 10 minutes to clear the old Code and then reconnect it before you try to program the new FOB.

Your english isnt perfect but i am assuming that you insert the key and it turns the ignition on but will not go any further to the start position if this is the case then it could mean a either a new ignition switch or the contact block on the back of the ignition barrel ,usual story if had vehicle in front of me i would soon know what is wrong but from afar ?' no idea try a drop of engine oil down the key hole and work the key back and forewards a bit and see what happens if that fails then ask a local mechanic to have a look for you .

If you don' having any wiring issues I would think you should look into the gauge.
The gauge has a memory. It is supposed to stay in its last position when the key is turned off. There would be no more power going to the gauge once they key is turned off so it can no longer read resistance to ground from the sender.

I would check your ignition switch. The actual part you put the key in. Try a new key first or your spare if you got one. Or try to put a tiny bit of lube on key. Like one drop..

Just an idea - could save a hell of a lot of time and money, but it depends how the key snapped if you can get the glue to stick...
you can put a tiny drop of crazy/super glue on the snapped part of the key, put it in the lock and make it so its perfectly touching the key, you can take the old bit of key out of the lock when it sticks and have a locksmith make you a new key from the two bits... cheaper than an ignition barrell.
use the tiniest speck of glue possible, use a hair or whisker to get a tiny drop of glue onto the key, put it in the same way round it snapped off so the two faces mate together properly - after 2 minutes pull out the key, if it is stuck give it a wiggle and still pull...
any glue that gets in the lock can be removed with nail varnish remover (acetone)
Dont know about how to remove the cylinder sorry...

if it will not unlock manually then it could have a link bar dropped off ,the plastic tie bit that holds then rots away or goes brittle and breaks allowing the rod to fall out