Shylock accepts the dinner invitation because he wishes to strengthen his advantage over Antonio. He has just caught the Christian merchant at a disadvantage since Antonio had asked him for a loan. Shylock's strategy is vested in the fact that he wants to get the better of Antonio since the Christian had repeatedly criticised, abused and mocked him. He hates Antonio.
Going to the Christian's dinner would strengthen Shylock's hand for they would surely believe him to be benevolent. Shylock does this not out of kindness but as part of a greater plot. He hopes on things going wrong with the merchant giving him power over him. Attending the dinner ensures that he does not lose any advantage over the Christians, no matter how slight, as he unequivocally declares:
...But yet I'll go in hate, to feed upon
The prodigal Christian.
Shylock had earlier mentioned that,
...I will buy with you, sell with you, talk with you,
walk with you, and so following, but I will not eat
with you, drink with you, nor pray with you...
when Bassanio invited him, meaning that he will do business and perform other menial activities with Christians but not socialize with them. He despises them too much for what, he believes, are the wrongs they have done him.
Shylock is not happy to accept the invitation because he bears no love for Christians and feels that their invitation is mere flattery. He does, however, decide to go for the aforementioned reasons.
The Prodigal Christian is Bassanio. Shylock knows that Bassanio is irresponsible and wastes all his money, hence the request for a loan. A prodigal is a wastrel. The significance of this is that it is Bassanio's irresponsible behavior, and his dependence on Antonio, that later leads to the dramatic events in which Shylock claims a pound of Antonio's flesh which, if successful, would either cause the merchant's death or inflict grave injury.
'I am not bid for love' means 'I have not been invited out of love (or affection)' and 'I am right loath to go' means 'I truly hate the idea of going (to the dinner).'
Shylock believes that his dream of money bags that night was a bad omen. He feels that he is being plotted against by the Christians or that some other ill will befall him.
Jessica and Lorenzo use Shylock's absence as an opportunity for her to elope with the Christian. She knows that her father would never allow her to have a relationship with a Christian and, in his absence, she escapes disguised as a boy, with her lover's assistance. In the process, she also steals some of her father's money and valuables, an act which would bring her father great distress, more than for the loss of his daughter.