using a buffer retainer

On the AR15/ M16, the spring is retained by the buffer. The buffer is retained by a pin located at the lower outer edge of the extension tube.

Yes it is complete, no it does not include a buffer or spring- and is rather pricey IMHO.

The bolt carrier would not return, another round would be chambered, and you'd likely have to replace a bolt carrier, buffer tube, buttstock (if using a fixed buttstock), and buffer spring (if the spring was in the extension tube). You could also cause injury to yourself if the bolt carrier ended up going through the back of the extension tube.

The HK 416 Pistol and Rifle are both the same in construction. the lowers are the same also. It is a scaled version of the HK 416/417 model (.223) only a dedicated .22 rifle. the place the Buffer-tube inserts is capped for pistol specs. But yes a buffer tube from a full length 416 .22lr should work. Be aware that by putting the Buffer-tube WITH a stock constitutes a short barrel rifle and is illegal unless you pay the ATF the $200 tax stamp and process the request to own through them.