If you switch to a Post Study visa, your citizenship clock starts over again and you are on a set timeline for how long your visa is valid.
No, depending on how you obtain your citizenship the clock for it starts from the moment you enter the country, not from your most recent visa. If you were to leave the UK and come back later on a different visa, the clock resets. The other thing you have to have is a visa without any time limits on it (ILR), but it doesn't matter when you get that as long as you have it in your posession when you apply. For example, someone could enter the UK on a work permit, get married at about 12 months into that visa, switch to spousal but not be able to get their ILR for another 2 years but then on the very next day they had their ILR apply for Citizenship.
The government is actively pursuing eliminating Citizenship based on the long-stay rule, or even just because you have continued to live and work in the UK. They feel Citizenship is not an automatic right.
To Bronco, you have to be in the country LEGALLY for the period of time required to get your UK Citizenship.