Just bought a Mark V 90 watt combo. Have some questions not answered in the manual.

I'm not new to tube amps, but given how nice this amp is, and how much there is to do with this amp I wanted to make sure I don't unknowingly damage something. Most of my questions were answered by the (fantastic) manual. But, I could not find answers to the following, and was hoping for your guys help

1.) Can the wattage selections (10,45, and 90) be changed on any channel, WHILE THE AMP IS ON AND OFF OF STANDBY MODE? Same goes for the triode/pentode and tube/diode switches on the back?

2. Can one switch from full power to variac power (or vice versa) without first shutting down the amp/putting it in standby? And if so how? It seems when I put the amp in standby, the power switch turns the whole amp off when it gets in the middle position, between full and variac power.

3.) This one seems pretty obvious but I just wanted to double check. Which wattage mode will be less taxing on the tubes and allow them to last longer? Like if I'm not at a gig and just practicing? 10 right?

Thank you. And I hope I didn't break any forum rules I'm new here ¯\_(ツ)_/¯

Let me first say that I can’t answer any of your questions with certainty but I will offer my 2 cents (I have had my MV for about 5 years now).
I don’t think any of the switch changes require standby or shutdown. Some may cause a pop which I always found uncomfortable or made me nervous but most of the settings you are discussing are things that you would most likely find out your preferred choice and then set them and leave them. For the wattage modes, I like the 45W setting unless you need the 90 watts for a lot of volume. The 10W mode causes some pops when changing channels (which the manual says is normal) AND uses one tube from each matched pair which causes uneven wear. The 45W mode uses one matched pair (inner I think?) and you can swap the inner and outer pairs after 6 months or a year or whatever depending on use and “extend tube life”.

Again, I am not a tech but do have some experience with electronics and tube amps. Hopefully some others will offer an opinion.

1) Yes
2) Yes, just push through the Off position.
3) I'm not certain really. I leave mine on 90W unless a venue wants to tame stage volume, then I drop to 45W.

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Q1 & Q2: Yes, as stated by my fellow owners above!

Q3: Review Pages 30-32 of the manual to learn exactly what tubes are used - and how they are used - in the various wattage configurations.

However, nothing is noted in the entire manual (I've reviewed it often in the past 5 months) about extending tube via wattage settings or variac power!

The only references to extending tube life are:

Use of the standby switch when not in use. (pages 4, 7, 10, 38, 39)

Proper Ohm Load (page 5)

Use of the internal fan (page 6)

Just one thing to add to the above, pretty sure i remember that the pop caused from switching between 10w to 45 or 90w is normal due to the architecture of the amp and is harmless. However, rather than suppressing this mesa decided to not add even more components to the signal chain (adding noise) because they believed that most users would not use 10w as well as 45 or 90w as a viable setup when playing live and switching channels.