Ah they are 'bolt on' ones, can't remember what Zafira ones are like but as you say just replace and remvoe any hassle, Worth £20.68 of anyones money!
Be careful you don't shear the mountings for the fuel tank straps, especially if they are also siezed, if they start to tighten (bind) up as you are undoing remember to tighten them back up again to clear the threads of moving corrosion and again get the penetrating spray out and lube them up well now before attempting to remove later...
Rear flex pipe may also be fun if the solid brake pipe isn't keen on coming out of the flexi??
In regard to the sump, cant you weld something on the sump plug to remove and then replace with a new sump plug rather than replace the whole sump? Seems a little extreme, and probably costly for the sake of a rounded sump plug?
Hope it all goes according to plan!