My only issue is that several changes have been implemented in a short timespan, and the result is that it feels it takes ages to find a game. Especially during midday European hours.

I'm always trying to see the game through the lens of a skeptical newcomer, and as it is now, it's just too fragmented. It doesn't have a good flow. I hope they implement more interactive elements asap. Before Season 3, at least you had it straight-on that the game is just a passive Find Match spamfest. Now it just feels that it has its legs in two boats.

Went a bit off-topic, but as far as your concern goes, I indeed don't see much scope for restraining rating range when the summoner levels are capped per league. If they want they can just add it in the Diamond/Champion leagues or just add a cap in Diamond.
As soon as I get in a new league, I get trampled regardless of rating, just because my cards are underleveled. Give it 2-3 days and other similar leveled ppl catch up and I'm starting to go up again.

Bottom line, I do agree with you in the sense that it feels this change has been implemented at the pressure of the high-rank players, who were tired of barely getting in Diamond and facing Top5 opponents. For lesser leagues, it didn't help with anything, it just made it:

1. more likely to face the same group of people
2. longer to find an opponent