yes the chain ring should fit but will lower your gear ration and you will likely need to shorten your chain. A wider selection of chain rings are available to order at your local halfords store

Halfords offer a free safety check on all bikes purchased after 6 weeks of ownership. I suggest you contact your local store (ask for bikehut) and book your bike in for this check, so the problem can be resolved on this bike.
If you do not get the expected level of service from teh bikehut colleagues (which you should do) then ask to speak to the store manager

Yes

Sorry, it entirley depends on your point of view as to which bike is better. In my opinion the x-rated decoy is slightly better

Bikes are not manufactured to a controlled weight. Minor variations in the weights of the numerous individual components fitted to the frame can accumulate to provide significant variance in the overall weight of the bike. An indicative weight of 15.6Kg has been obtained by weighing a sample of this model, but as explained above individual bike weights will vary.

Dues to the style of BMX they are very small. This effectually a full size BMX. Although it may not be the most suitable style if bike for long rides it will be a suitable size BMX for you.

Every bike on the website features a 'Choose the Right Size Bike' link, which advises of the correct size frames for your/ your childs personal measurements. This will ensure you find the right size bike regardless of age.

The performance would certainly not decrease. The chains that come on the x-rated's are fairly decent so putting on a Taya chain would up the quality a little. You would most likely not notice any difference.

The only advantage of fitting a tensionser would be keeping the chain tension for a longer period. It is common that BMX rear wheels need to be pulled back often to keep the chain tight. Therfore the tensioner would decrease the amount of times you have to do it. And make it easier to pull the wheel back.

Yes, front and rear stunt pegs are included as standard.

It is a 3-piece crank on the hustle. The nut on the LH crank arm near the Bottom bracket holds it all together. Remember to remove the pedal first before disassembling the bike!