1. The range of the boiler is irrelevant I'm afraid. The testing must be done at full output, and your pipework will not cope with a 27 kW load.
2. As already stated, the absolute maximum your current pipework can support (subject to survey) is around 24 kW, and given the number of bends / elbows I suspect it won't really even support that.
3. If any of the long lengths on your diagram could have larger diameter pipes installed without major disruption, you might be able to upgrade those to 35 mm pipes and leave the remainder as 22. But this is a calculation which MUST be done by a Gas Safe Registered engineer who has full view of the site.