1 Your Pension is part of your terms and conditions. Can a company unilaterally change those without your permission – I think not.
If so it is up to the company to negotiate with you as to what you would accept.
2 The current share price reflects the current pension scheme. Shares in companies are regularly traded so the shareholders are fully aware of the current situation . It might be that the shareholders would be quite prepared to dilute their shareholding to allow current employees to exchange part of their pension for additional new shares on the basis that if the deficit was cleared the returns on the shares would be greater.
Whatever you do, don’t give up your pension for the benefit of others because one thing that you can be sure of is that the directors of the Company have an incentive scheme tied into share price, so reducing your terms and conditions would hugely enrich the directors.