Not at all certain about non-Bitcoin sidechains, and not even 100% certain about Bitcoin ones, especially because I'm only "familiar" with Bitcoin sidechains because of Rootstock (and maybe if you consider Counterparty a sidechain?) but KingZee probably summarised it in his first line, you absolutely need to secure it on its own. As a general incentive to do so, normally miners are given option to merge mine the sidechain with Bitcoin.
Correct. What i was talking about is re-org within a the sidechain's merge mine without the need to re-org the main chain. Which was described in here.
http://www.truthcoin.info/blog/blind-merged-mining/That's beyond my depth of understanding unfortunately. My own experience with mining (POW anyway, I've done plenty of POS staking which didn't take a single effort other than opening the wallet) is with already dead chains. At any rate, it's more than 2 years since that's proposed. My guess is by now something's been done already similar to it... or with Lightning well in midst of rollout, the motivation behind the post (scaling) is no longer primary?
btw, you're an old hand...