What's been said about the header tank levels is correct, some show right on level others usually low' It's not a design specific tank and it would seem slight engine variations make a difference in cold and hot levels!
Did it have any of these issues prior to the modifications being made?
You have had yet another aftermarket header tank option fitted and perhaps its expansion capacity just isn't great enough. If the cap isn't sealing then that will change the boiling temp as will its pressure relief function/capability. you seem to have all those associated symptoms. Is the header tank position now lower as its repositioned on the bulkhead and filling with fluid prior to full pressurization of the system. Could be that the thermostat is also not functioning correctly or is of the incorrect operating temp type, ie. replaced as an incorrect item. Is it actually boiling? if so, both the cap seal and venting pressure are indicators of this scenario and the thermostat seems to not be opening early enough. The temp gauge in the car might show 100'c but is it? a temp target of 90'c is the desired so ascertaining the true temperature is required. Have you got the old header tank, if so refit this and see what happens. might answer a few questions.
In my time I have seen all sorts of things from paper towel packed into the bottom hose outlet to stop it dripping on the mechanic working under the car and then left in when reconnected. Blanking plugs left in rads and hose outlets to faulty new thermostats. There was a time when you had to check every new stat prior to fitting, both function and temp!
Hope you get to the bottom of it soon!
Head gasket failure, you'll get white exhaust gas like a kettle boiling steam! and I'd think a greater loss of coolant in the system.
