When did carbon monoxide come into it? Looks like a typo for CO2 to me. In any case CO is fairly reactive and soon combines with oxygen to give CO2, aided by UV. That's why CO from petrol engines doesn't build up in the atmosphere
The water in the F/E tank doesn't circulate through the system, I doubt whether corrosion is any different from using an expansion vessel.
With an F/E tank there's no need to check pressure and top up periodically, it looks after itself. F/E runs at lower pressure, so less likely to cause leaks...
Mainly up-front cost, including possible need for a bigger gas pipe to meet the pressure drop regs. Might be a little less efficient as it will be working at a big turndown.
Does it have to be a system boiler? A heat-only is easier to troubleshoot in event of a problem. It also allows the option...
Highly unlikely they're quoting a single pipe system. If they are, tell them where to go!
If the boiler and HW cylinder are both to go in the airing cupboard it must be a big one.
WB have quoted pipework 15 and 22mm, the other quote doesn't say, but worth making sure they heven't based on 10mm...