Interesting thread;
I don't think you can wait for 'the first flood' as kelts will go when they're good and ready (as has been pointed out) also you can't start the season on the first flood, people have businesses to run on the basis of these dates: hotels, tackle shops, ghillies, guides etc. You can't run it on a 'moveable date'.
Quite often the Kelts decend on the Welsh Dee in April, so there's a big overlap between the start of a new season and when the kelts go back to sea. This is common on many rivers, and I've seen springers in amongst kelts up at Callander on the Teith, on the North Esk at Kinnaber and the Tweed at Upper Hendersyde in March. Especially true on rivers where there are several distinct runs of fish, including 'winter salmon'.
Secondly the actual damage caused in catching a kelt; there's always a risk of a deeply hooked fish, to be fair the fish are on their way downstream and many will become seal fodder, but they're going to be back in the sea feeding within a matter of days/ weeks.
Also if you are using standard spring fishing gear, most anglers will see it's a kelt and be able to 'horse' in the fish in double quick time- if it comes off, then it doesn't matter anyway- what matters is a quick release.
In Scotland I've seen anglers (once a kelt's been identified) get the fish into the net in about 30 seconds flat, and away again within a minute of hooking them. This doesn't do much harm- there's no protracted scrap.
To be honest it would be good if you could avoid catching kelts, but they put a bend in the rod and tend to 'go back well' in my experience. I don't think this incidental catching does a great deal of damage and wouldn't put my off going after a springer.
Andy