"But I do ask my good friends who voted SNP twice why they did not consider RISE or Green?"
I'm sure you'll get a range of answers.
As has already been discussed on here, people are clearly free to vote for X twice (wasn't just the SNP calling for it).
I suspect there will be years of academic analysis of 'what if'.
Whether the SNP really believed that voting for them twice would give them more seats is unclear. I think in some regions probably, others almost certainly not.
On one hand it's reasonable to think some SNP voters would/should (and did) vote for either of the two parties in favour of independence - but individual voters will have decided they don't like their policies enough. There must be people who voted SNP and also for one of the pro-union parties.
There will be criticism of the voting system from all sides, but generally it works as intended, though many people still don't entirely understand it.
Whether individuals or parties misrepresented (wilfully or through ignorance) how it works is another issue!