Far more difficult than you can imagine. Old Rag is equivalent to Tiger Mt, basically just a hill. Hood is steep, icy, prone to rock and ice fall, is relatively high, has far more elevation gain, has no trail other than a boot path in the snow. People die on it all the time. Beyond the physical and technical demands, experience and judgment is required. That is perhaps the most valuable asset a professional guide supplies, good decision making skills.
Yes. You are better off going with a guide. Timberline is reputable, never heard of the other one.
Probably, but conditions are different from year to year and can change quickly. I personally won't climb Hood later than May.