Tapping land for mana with free cast and X in cost
No, in both cases, you cannot tap additional lands to pay for the X. X, if it's not specified on a card, is always considered to be zero. In both cases, the spell's cost is not being paid for, it's being replaced. Galvanoth and Jeleva aren't saying "Add mana to your mana pool equal to the casting cost", they are replacing the cost altogether. So you can't add anything addional to the spell.

Relevant Gathering rules for Galvanoth:


  If the card has an X in its mana cost, you must choose 0 as its value.