you can easily win in PVP just by double checking the medal values. the rule or thumb typically is you should see a lower ammount of medals won then loss before a matchup and it's almost always a guaranteed win. besides that also look at the enemy tank lineups as a double check. don't de rank, that's a scumbag move that messes up the pvp system. you should also check and ensure your technology for your ammunition and other things like manual are up to date in the S&T. 9/10 that will solve a lot of your problems once you upgrade everything you need to.

getting silver is actually pretty easy besides pvp. early in game accomplishments usually just pass them out. it's a good idea to also keep on top of updating your headquarters and silver vaults. spamming previously S ranked missions is fast and easy for farming up codons or silver if you need, plus you get equipments to boot.

as far as tank lineups go, having a high detection scout and a heavy tank that eats alot of damage is a good start to build around. this is reference to your number 4 question also, she's an awesome early game heavy. grow her asap.

spending good to unlock the bwmg was a very good idea, you made the correct choice. owning that building becomes extremely vital in the day to day routine of playing this game.