These are my opinions based on limited experience:
Q1) Yes, the modern technology found in some of these high priced attenuators does a great job of preserving tone. Most guys using them are recording direct into their computer so it is difficult to determine how that would sound in a room directly through a speaker.
Q2) One reason that an attenuator might wear out tubes faster is because you will be pushing the output tubes harder than you would if you weren't using an attenuator.
Q3) The higher priced stuff gives you more options for signal routing and computer interface. I was recently looking into the Fryette Power Station.Listen to how crazy this is. It has a 50W tube output stage that you use to re-amp your attenuated signal from your 50W amp.

But there is some magic that happens there...check it out:
https://www.fryette.com/power-station-integrated-reactance-amplifier/ And have him check out this:
https://www.fryette.com/fryette-power-load-ir-pl-ir-in-stores-now/Q4) IMHO, the only way to get the true Plexi tone that everyone freaks out about is to build a Plexi and push some EL34s. Then knock it down with a competent load. Get some low efficiency speakers and point them at your head.
Here's the biggest thing I have learned:The tone that you perceive in the room is more about how the volume comes out of the speaker and hits your ear. i.e - if you don't push the speaker, you will be missing some of the TONE. Highs have a lot to do with TONE and how we perceive them has a lot to do with our ears. A resistance-only load dampens HF response. The more expensive boxes add switchable capacitors and/or speaker motors to overcome the muffled sound associated with a cheap resistance-only load.
Hope that helps.