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For navigation in particular you'd use capital D to discard the current buffer and return to the previous page. There's also , (comma, back) and . (period, forward), which non-destructively cycle through the stack.
(Well, it's really a tree, but the UI mostly treats it as a stack.)
IANAL, but I have been around the startup scene long enough to immediately spot the red flags here. It seems like there is an explicit strategy at play here to discourage former employees from exercising RSUs, and you will likely find a strong legal argument against these terms (i.e. prepayment and forfeiture) unless you explicitly agreed to them. To echo what will probably be stated at least a hundred times on this thread - talk to an attorney.