Happy Chinese New Year folks! I'm here to publish the answers.

A) 2C-2D-3NT-4C-4NT-6NT

2C: Any 23+ HCP hand OR 19+ HCP with 8.5+ tricks
2D: Any 0-7 HCP hand
3NT: 25-27 HCP balanced, no 5 card major
4C: Gerber (I realised this should be Stayman. In future, sequences like this will be Stayman)
4NT: 3 Aces
6NT: To Play

B) 1D-2C-2H-2S-3NT

1D: 12+ HCP, 3+ cards, no 5carded major
2C: 10+ HCP, 4+ cards, may have 4carded major
2H: Reverse - 16-18 HCP, 5-4+ D-H, gameforcing
2S: 4+ S
3NT: To play

*2S isn't artificial because 2H is already gameforcing (16+10). Alternatively, you might want to play it as asking for stopper to play NT since partner is unlikely to be 5-4-4-0.

C) 1S-1NT-2H-2S

1S: 12+ HCP, 5+ S
1NT: Any 6-9 HCP hand, no 3card S
2H: 12-18 HCP, 5-4 S-H, non-forcing
2S: To play, 2card S

*There could be two reasons for bidding 2H. One, opener might have genuine game interest if responder has maximum values. Two, opener does not want to play in NT. In this case he likely has more than 5-4 in majors, perhaps 6-4 or 5-5. Since responder does not know which case is it, he should strive to keep the bidding alive lest it's case 1, unless he has minimum values (6 HCP) and knows game is impossible. In this sequence he shows a preference for spades over hearts.

D) 2H-3H

2H: Preempt, 6card H.
3H: Further preempt, to play.

*Remember that conditions for preempting (no 4card major, void etc.) are to be adhered when in 1st/2nd seat but are more flexible in 3rd seat since no game is possible and the objective is to disrupt opponents bidding. Also note that 'preempting' in 4th seat shows 12-15 HCP and 6+ cards. Reason is if you are weak can just pass out. 3H in this case merely eats up opponents bidding space. Check out the vulnerability when preempting too.

E) 1S-2C-2S-4S-4NT-5H-6S

1S: 12+ HCP, 5+ S
2C: 10+ HCP, 4+ C, may have S support
2S: Waiting
4S: 13+ HCP, 3+ S, to play
4NT: Blackwood
5H: 2 Aces
6S: Sign off

*2S is 12-15 HCP with 6+ S only when responder responds at level 1. At level 2, it's a waiting bid. Bid 2S when you don't have the following:
1) 12-13 HCP, balanced (2NT)
2) Second suit to show (shift/jump shift)
3) 16-18 HCP, 6+ S (3S)
4) 16-18 HCP, 5 S, balanced (3NT)
5) 15+ HCP, 4+ C (3C)

People might do a waiting bid even with 5). Waiting bids do not promise anything more. Examples would be 6+ S, 12-15 HCP. By standard convention, 3S after 2S shows minimum 10-12 HCP with 3card support (not 3+ else would have 3S after 1S) wherelse 4S shows 13+ HCP, 3+ S and is essentially to play if opener does not have much more. However, if opener has say 19 HCP with 7 S he will have slam interest and would do the waiting bid as well, hence the Blackwood bid. However more often than not will you get a hand where slam is so visible. Takes a decent amount of judgement for a slam try when you have an intermediate hand (maybe 16 HCP). Good judgement comes with lots of practise :)

Cheerios.