You can avoid the Siskiyous if you go 101, but it will slow you down. The drive up the coast is great though, southern part is not nearly as busy as north of Florence. If you do head that way I'd head back to I-5 via Highway 38 (Umpqua Hwy) not Highway 126 - Hwy 38 is a much smoother easier drive, 126 has been in need of serious widening and smoothing out for over a decade.
Used to be if you bought chains at a Les Schwab Tire Stores you could return them for a refund if you didn't use them. I couldn't verify this on their website though, but it might be worth a call. There have stores all over the NW and a large number in CA too.
That time of year in Oregon can be a real mixed bag. We've spent spring break in Florence OR for the past 10 years and I grew up in Eugene. There are times the weather is wonderful, 60's no rain and there are times you wonder if the rain will ever stop. It's totally hit or miss, be prepared for rain and you'll be happy if it doesn't.