Upvote:1
As to your question #1: Yes, you do need to show proof of provenance of all funds that'll support the trip.
One of the applicant's jobs is to convince the ECO that the funds to be used for the proposed trip were legitimately obtained, whether by the applicant or by the applicant's sponsor(s). Otherwise, the ECO can conclude that the funds were not obtained legally. It's not up to the ECO to prove illegality or impropriety; it's up to the applicant.
As to your question #2: No, business invoices alone are far weaker evidence. I would absolutely not rely solely upon them. The reason is that invoices are wholly under the applicant's control, and what's shown on an invoice may be a complete fabrication. The ECO doesn't know if your parents are truth-tellers or liars; the fact that a number is shown on an invoice will not be persuasive.
For a lengthier discussion, see the section "Provenance of Funds" in answer to this question.