On 'good deeds will go to waste if there is nothing after death' I understand that an Imam has said that the real loss is if you've no good deeds and there is a day of reckoning. Does this imply that it is impossible to prove there will be such a day?
The evidence of the Day of Judgement is more than counting. The sentence which you quoted is narrated in a debate between the Imam (AS) and a non believer who denied the Hereafter, so Imam was telling him that either there is Hereafter or not, so if there is a Hereafter, the believers will win, and non believers will lose, and if there is no Hereafter, the believers will not lose anything.
Wassalam.