From everything I have read I am not yet convinced this a head gasket problem. I would really like to see those spark plugs. The one question I never saw asked was, what is the condition of the oil? You are adding coolant every day. Is the oil okay? A blown head gasket will often allow water into the oil. If this happens the oil level will rise and look like a latte. Also a blown head gasket will cause the engine to run rough. Is the engine running smoothly?
Are you adding coolant to the overflow bottle or to the radiator? If the over flow bottle isn't part of the pressurized system then it wouldn't show up as a failure when the system is pressure tested. If the radiator itself is never low, the overflow bottle might have the leak. it seems unlikely, but still possible.
There is another possibility. a non pressurized overflow bottle is designed to be filled to a set point. If it's filled to the top, the system can vent pressure into the overflow bottle causing it to spill over and that would show up as a coolant loss.
If you are adding coolant to the radiator regularly then the cap could be installed wrong causing loss of pressure.
The people who checked the system did they check the radiator cap? If you have always had this problem it could be the cap may fit, but it is designed for a lower pressure system. Did they just pressurize the system for a minute or did they put it under pressure, and checked to see if it was holding after say a half an hour? You might have a very slow leak, only happening once the engine is warm and metal parts have expanded, . Such a leak wouldn't happen while the vehicle is sitting because the system is cool and not under pressure. Such a small drip could seem invisible because it can either evaporates from engine heat or is dispersed onto the road.
I don't remember mention of how much coolant was being added and how often.
It's unlikely, but there is a chance that the cooling system is leaking into the transmission through a broken cooling coil in the radiator. Have you checked your transmission fluid?
Until you get the leak fixed you might want to add straight water because there is no point in spending money on coolant you can't keep in your system. Just bear in mind water can freeze and crack your engine block if you are in a area where temperatures drops below freezing. Straight water will also allow the system to corrode so fix the problem and use proper a proper water/coolant mix as once it's right.
If it really is a combustion chamber leak, (blown head gasket, cracked or warped head) then any 'head gasket in a can" is at best a bad gamble. I have seen them work, and I have seen them fail. Even it it does work, your engine will never be rebuildable and it's time to start saving up for either for a new engine or a new vehicle.
A new engine is far better than a head gasket, but money is always always a factor. I dropped a used engine into Honda and it ran like a swiss watch for years until I sold it, but that was a chance that could have gone bad.
I have never had a problem with Bars Leak for sealing small pressure leaks, but it is at best a stop gap and not a fix. There are a lot of "stranded in the desert" tricks that do the same job that will work. Some ground black pepper or fine saw dust in the radiator can block a tiny leak for as long as the engine stays running, but once you shut it off it will likely fail. A raw egg into the radiator will seal a small leak. Keep in mind that these stop gaps will also cause blockages in cooling system that at the very least will require serious flushing. A clogged system can cause overheating and engine damage so these hacks are 'emergency only and at your own risk' for when all other options are exhausted. A tow truck and a minor repair costs a lot less than an engine, so choose wisely.
