The display (and theoretically the computer) just use rpm/mph to update the gear. I've had it mistake 5th and 4th one time while I was on the highway, so much so I thought I had put it into 4th, clutched in - and then it fixed itself.
The car already has a clutch & neutral safety switch, every modern manual car is required by law to have them, if you wanted to setup a remote start, you'll have to either fool it into thinking the clutch is engaged when starting while the car is in neutral. Clutch switches are usually on the pedal, neutral switch is on the transmission- typically in the turret.
The neutral switch isn't used when starting - it's for disabling cruise when pulling it out of a gear. Now for the clutch switch - there sometimes are two, one for disabling cruise, the other is the safety switch. In theory you can make your own using an Arduino, just act as a man-in-the-middle with the clutch safety switch, confirm it's in neutral with the neutral switch, and have it send a start signal... though now thinking about it, you have to have the key in the car else the computer won't disable the anti-theft. Might be worth seeing if the CVT remote start system can be retrofitted to work with the 5MT.
Well first thing to note is there are two switches in the pedal, top and bottom. Bottom for starting car, top for disabling cruise control. The gear will not display until the pedal is fully released, if you accidentally rest your foot on the clutch (or not accidentally), the gear
should go blank and stay blank
. But it does not always go blank, and until you let off the clutch it will not update again so If you are fully letting off the clutch and getting wrong gears or staying blank your switches, connectors, or wire could be faulty.
Second I didn't mean that each gear had a wire simply that it was wired in some way to show gears, but all we really need is 1 wire. The transmission neutral position switch is labeled T12 on this diagram, though there's other places to access it. Also 1 more switch by law labeled T13, a reverse switch for the backup light and camera. The other two plugged into T12 and T13 labeled T1 and T2 I assume go to selector fork switches. Personally I've never had any issues with it showing wrong gear by the time my foot hit the dead pedal though though I could see it happening if neutral switch malfunctioned during a shift. No issues with clutchless shifting either.
Personally I will be trying to do the OE route. The remote unit does talk to the BIU to check if parking switch is active over CANBUS, but 5MT BIU should know it's a manual so that might be the catch 22, or it might not care if parking brake is on. Not hard to short bottom switch to trick it into think pedal pressed for testing, at least you don't lose cruise control like my old Honda, but you do have to be careful about accidentally starting in gear when using turn key/button. But all the switch does is cut power from Pin A25 of ECU to starter relay for turn key, push button uses keyless module. Of course there are safer ways to do this with the right electronics than just disabling.