I can barely read your post but what I can see is your suggestion blizzard switch to 15 man raiding which is something blizzard has said down right no to in the past.
The primary issue with you not able to hold on to raiders seems to be your are keeping a massive raid core.
That's 15 people to field a 10 man guild. Back when I raided 10's (about 3 weeks ago). We fielded a 12 man roster for most of our clears and it honestly never held us back (see top 20 US). The main issue with stacking your roster this big is what you pointed out, all your members can't raid all the fights. Normally people won't have an issue sitting out for 1 or 2 fights per week, but when they are out more then they are in, its a problem.
When you consider roster size you should really try to 'scale' your group up to 25man size. 15 members is a bench of 5 half your overall raid group. This is the same as a 25man guild having a bench 12 member large. This means if you were running a 25man with the same bench size you would have 37 raiders.
The issue with a bench this big is unless you are a top guild nobody wants to sit. Nobody pays 15 bucks a month to be told 'Not this week billy'.
You don't need 3 tanks, 2 tanks will really work the 'imbalance' between tanking specs is scarcely noticeable. At anywhere under the top 100 ranks. Really if you have 1 shield tank (Warrior or pally) and 1 non shield tank (Dk or druid) you cover most your bases.
4 Healers is also over kill. You can get away with 3 (at the bare minimum). We ran resto shaman, druid, and holy pally. Then when we needed disc heals super bad we made our shadow priest respec.
8 to 9 dps is just wrong. You don't need to cover every class, because if you try to then your every patch 1 member will always fill the role of bench warmer. Its way easier to have your tanks keep a pve dps offspec for fights where you only need 1 tank.
[Today 06:48 PM] Ion:swimming in a natural body of water ISN'T acceptable...it's momentarily tolerable