Altogether I think the changes/non changes sound all right.
Rebalancing the Market prizes is a good idea, last War they were slightly unbalanced even though our team had an advantage through this.
I also like the idea that pillars/pends will be unlimited, that way the vault only focusses on playable cards. Organising the vault takes a lot of time and makes War tedious, I'm a big fan of any simplification.
I'm actually particularly fond of the Sideboard. Many matches in War have a pre-defined outcome and you just sit there to get beaten three times in a row without any hope of winning. A small sideboard, not enough to turn a rush into a control deck but 2-4 cards (I'd personally like 4 but I agree it may be too much), would make things a bit more open.
It would also make player skill more important, therefore subbing should be restricted or people who are allowed to sub should be pre-defined by the War masters.
The importance of player skill would influence the auction as well, as the difference between a "cheap" new player and an expensive veteran would be even greater.
I'm not sure it would make deck building much more time consuming or difficult as it would give a bit more freedom instead of wrecking your head between two types of decks and eventually just gambling.
I understand mrpaper's concerns regarding new/inexperienced players and regarding the fact that the Strategist role was OP when it was used with a sideboard in War#6.
Regarding inexperienced players, the reduction to 5-man teams may help a little there, also most players who are still around the forums have at least some knowledge of the game through various pvp events, and complete noobs generally don't have the card base to join War anyway so I'm not sure we have many people around who couldn't handle a sideboard.
The Strategist's sideboard in War #6 was overpowered because only the Strategist had one, that did indeed give an unfair advantage to pretty much every other role including the Generals, for this not to be the case, everyone should have one imo.
I really think having a sideboard would make War more exciting and I'm a big fan, but again, I understand the concerns people expressed and they may be right, so Zawadx' suggestion to create an event testing this system sounds like a very good idea to me, it could give us all insight (and fun with a new and unexpected pvp event).