This is a really hard call and ultimately it's very subjective.

Nik's amps are designed to be exact-spec'ed clones of the original amps.  He seems pretty true to the original schematic, but uses his stock sourced parts and may make a tweak here or there for reliability.  Will it sound exactly like a Marshall, that's probably going to be up to your own ears.

Perception is a weird thing.  How your mind processes what you hear is based on how tuned in in to the proceeding tones.  The exact same amp can sound too bright or too warm depending on what you were listening to right before hearing it. As a result, when comparing two any similar two amps, their differences relative to one another going to stand out and be prominent.  But throw maybe a dozen amps into the mix and you mind works different, it starts to hear what is similar between all the amps because it  no longer wants to process to minute differences, throw enough Jubilee clones at your ears in one sitting, and they'll all start sounding the same. 

This carries over to you other questions.  While I think the Jubilee does NOT sound like a JCM800, there are those who feel that with the gain turned down, it does.  They are less oriented toward tone and more toward does it get the right amount of crunch.  But they are very different preamps the HI Input on a JCM800 adds an another tube gain stage before the preamp, on the Jubiilee switching to Lead only adds more clipping diodes.  They may look like they have the same number of tube gain stages, but the Jubilee's are actually being used as effects loop buffers as well (at least in my rather blurry schematic I'm looking at).  It makes up for that with the diodes. 

IMHO, I think you'll find they both sound like a Marshall, and beyond that it's a matter of nuance.  I think the Jubilee is a bit buzzy and bright, but that does help it cut through on stage, where as the JCM800 offers a warmer tone and a bit smoother saturation by using a more Mesa Mk I style cascading gain stages preamp.

If you like the sound of the Marshall Jubilee (or even JCM900) over the Marshall JCm800, than you'll prefer the Ceriatone version more as well..and vice-versa. 

That's a hard question to answer, neither amp is designed for low volume.  They'll reach their 50 watts RMS pretty fast, and part of that Marshall sound is the power amp crunch.  I wouldn't recommend either for playing around the house.  At least not without some mods. 

The clipping diodes in the Jubilee are there for just that, to clip.  To get the cleanest tone, you would have to be on the rhythm channel with the rhythm clipping off.  On a JCM800 you could plug into the Lo channel is still get JMP-esque cleans and overdrive.