I'm going to answer your posts backwards:
1) keep the
citadel siege if you're not focusing on the life gain anymore
2) I haven't purchased cards from that online site, I mostly trade or by cards off my friends.
3) With the Ajani and the (+1/+1) counters all your creatures will get a (+1/+1) counter for EACH token then +1 more token because of the
hardened scales. So my math is this: the enchantment
hardened scales gives each creature an extra +1/+1 counter so instead of any creature getting 20x (+1/+1) they'll be getting 40x (+1/+1) counters. That's 800 COUNTERS! Since your
enduring scalelord gets a (+1/+1) counter every time one or more counters are placed on another creature AND it'll gain double that because of
hardened scales then it'll also gain an extra 40x (+1/+1) counters. So your dragon looks like (if I did my math right) will be gaining about 840x (+1/+1) counters.
4) Also yes to the alter of the brood, they would lose 20 cards.
5) The way that
phyrexian obliterator works is this: YOU sacrifice permanents equal to the amount of damage you DEAL to it. So if your
fleecemane lion is a 4/4 and kills the other monster on contact then you still have to sacrifice 4 Permanents not 5.
6) I believe that you don't. Reason being, you only sac permanents if you
deal damage, but if you outright destroy it then there is no damage.
7) If you cast
arrest on a creature you can still use it for other cards' CONVOKE costs. The creature can still gain +1/+1 counters and be used for other creatures activated abilities (similar to how
zulaport chainmage taps other ally creatures). Also an activated ability is something that you have to PAY A COST to use. For example:
ulrich's kindred has an ACTIVATED ABILITY because you have to PAY to use its effect.
your
servant of the scale would enter the field as a 2/2 creature.
That's all I've got for now, I hope it helps. If my math is off on the first one then I apologize in advance, it gets tricky when your stacking effects on top of one another.