One piece of advice, given that you are losing pressure – a possible cause is a leak in the pressure relief valve. The drain for this should be an external open pipe somewhere close to where the boiler is mounted. Check for dripping water; using a rubber-band, secure a small clear plastic bag on the end of the pipe (to collect any water) – there should be no leakage of water from this pipe.
If the boiler proves un-repairable, I would say that the only realistic route to obtain redress is to obtain an expert report on the boiler, then using that report, sue the installer through the small claims court if he won’t refund you.
But there are significant risks to this strategy, the expert’s report (which you will have to pay for) might not conclusively lay the blame with the installer/boiler. And even if you were to win a court judgement, it could prove difficult to enforce it.