Will companies tell you what stock is actually worth at different valuations when you join? This seems like far more valuable information than the valuation as of the last round.
Can you negotiate with the acquirer for a retention plan? Unfortunately, as an individual the power balance is very much not in my favor. Has anything like a pop-up union ever been used to get a seat at the table on behalf of the employees?
Your compensation and retention is between you and your boss, whoever that is. You are unlikely to get good results by skipping the management chain and asking the acquirer for special treatment. But you should be able to have professional discussions with your boss about your value, and negotiate from there.
There is little chance that you can get a seat at the table for the actual acquisition. That simply isn't how acquisitions work, not unless you already hold a role where the leadership includes you in strategic discussions.
In short, talk to your boss about all your concerns.