You had the answer on the first page. All the rest has been useless drivel. The expansion vessel will be completely flat, I'd bet money on it. The pressure gauge you're reading is on the inlet as far as I can tell so the rising pressure won't be seen as it's after a non-return valve. TPRV or PRV...
So if that's the case tell me why the overheat thermostat (usually) cuts power to the zone valve and closes it? If you're G3 qualified you'll know this ;) .
Manually opening the zone valve is effectively bypassing a safety device and potentially dangerous. This is why unqualified people shouldn't be giving out advice to unsuspecting people.
These do have a built in bypass but if it's noisy you might need an external one to reduce the noise. Need's to be tee'd in on the main flow and return but before any zone valves
Some new boiler flues can be fitted from inside by using a slightly bigger hole. The rubber seal is folded in then expands when it gets outside before pulling it back to the wall. You can then have the inside properly sealed to make it rigid.
You should be able to get the lid off with some manipulation. Never found one yet that I couldn't...
Keep it well above the water level. As long as it's properly through the lid you shouldn't have any problems with splashing as it should only be discharging water in a fault situation anyway.
I see oversized boilers on a daily basis. Perfect example today was a Vaillant 637 (37kw) when a 624 would have done it perfectly well (12 rads). The difference is that the 4 series fires on high rate where the 6 series starts on low. If you oversize a 4 series it will cause more issues than the...