1) Because it was a dynasty and monarchies tend to go straight for the eldest child for a reason. A child ruler is never a good thing, it leaves a vacuum at the top and however well meaning the regents are, people doubt their loyalty and it is hard for them when the child approaches adulthood but there is a chance of wise regents keeping things secure.
Change the heir and chaos guaranteed. See Sun Quan, Yuan Shao, Liu Biao, Emperor Ling. It leads to huge questions of legitimacy if the child is not chosen, other family members will respond to not being chosen by aiming to seize control, it undermines the sense of legitimacy and court splits into factions with some officials perhaps aiming for the throne themselves.
I mean if it was me I'd have even given the throne to a close friend if needed be.
You pretty much inviting civil war, every political rival will rebel and your family who just lost the throne and your wife.
2) I doubt when Sima Yi became regent, he intended to seize power. It is hard to know quite what his intentions where after he did though and he left his sons in a position to seize power and I can see why they would.
In terms of Cao Shuang, it is hard to know. The Jin historians certainly accused Shuang of horrible government but they were not going to go "Shuang was an able politician and his regime was a progressive if controversial one." Cao Shaung showed political talent in getting better of (though lacked ability to fully destroy them) the wily Sima Yi and the clever Empress Dowager for a time, he had great talents serving him as even the histories admit, his government attempted major reform but details are limited. Sima Yi vetoed one reform he admitted was a great idea simply becuase Cao Shuang's proposed it.
The coup could have been number of reasons. Seizing power, personal ambition from a man who had lost power, Wei loyalty with concerns at Empress Dowager losing power (the timing so soon after her fall does suggest she played a part in this) but there was also idealogical divide. The rivalry became Sima Yi the Confucian gentleman and protector of moral values vs Cao Shuang and his neo-deoists (he didn't employ many but perception matters) with reforms that were not in the gentry's favour, that may have been too radical for comfort.
“You, are a rebellious son who abandoned his father. You are a cruel brigand who murdered his lord. How can Heaven and Earth put up with you for long? And unless you die soon, how can you face the sight of men?”