Always on radiator

Combi boiler systems need a by pass radiator & it’s usually where the room stat is positioned Best solution is a remote programmable room stat

Your radiators need to be balanced properly, sounds like the hallway radiator is being favored for circulation so close the lock shield down a few turns but don't close it completely and then see how the front room radiator performs- remember rule of thumb- water will always choose the easiest route of less resistance back to the boiler which is why on an unbalanced radiator circuit you may have some rads hot and some cold- balancing the hot ones will allow the others to get hot.

Difficult to 100% answer this question without seeing in person. Generally speaking, the control radiator (the rad fitted in the same space as the room thermostat) needs to be left fully on so that the room-stat operates. If you starve the room-stat of heat, you will cause the heating to stay on longer,thus baking other parts of the house. Provide too much heat, the heating will cut out early leaving other sections of the house cold. Consider these points: 1. Balancing of radiators throughout to allow for even distribution of heat. Shut down the upstairs radiators to a trickle. 2. Maybe your lounge rad is undersized? Fit a larger radiator? 3. Is the thermostat positioned correctly and far enough away from the radiator? Combi boilers shouldn’t need a bypass unless the manufacturers specify since the boiler dissipates its heat via internal circulation. The design principle of having 1 radiator permanently fully open concerns the operation of the room thermostat only.

Sounds like the rads require balancing to ensure they all heat equally. If one heats up too much it steals the heat from the system leaving other rads cooler