As someone who has been on Rainier several times, including while very much inexperienced (and without a guide) I will make the following brief points. First, a climb of Rainier with a guide service, by one of the non-technical routes, is totally doable and reasonable in terms of safety for a person new to mountaineering, provided he has a sufficient level of physical fitness. Without a guide I'd never recommend it to a beginner, but since that's not being discussed, it's irrelevant. Now, I have very limited experience with guided mountaineering, but I do know that they vary considerably in terms of how much instruction they do, etc. That being said, if there is any advice I can give, it's that the more you already know, the more you will be able to learn from your experience. In other words, if you go into the trip not knowing how to tie a figure-8, you will be forced to take up time and energy doing that, at the expense of other valuable skills that you might have. Also, I've already mentioned the physical preparation and I'll say it again: the more, the better. More than anything, a guided Rainier climb is a test of stamina and endurance. You are gaining 9000 vertical feet usually in a day a half, with just a few hours of sleep in between. On top of that, you are going from sea level to over 14k feet without a chance to acclimate. Now, training will NOT prepare you for the altitude issues, but it will at least make the rest of the experience more worthwhile. Of course training by hiking (preferably with a pack) is the best way, but if that's not an option due to time constraints, regular cardiovascular exercise is good, too.
Having briefly read your posts, one thing I would caution against is underestimating the mountain, even if the trip is guided. That's not to discourage you from going, but rather to have the right mindset when you're there. It's always better to prepare for the worst, and that have it turn out to not be so bad. However, that's not likely to happen...having been in your shoes, more or less, believe me that you have no idea how big and serious Rainier is until you have climbed or attempted to climb it.
Anyway, whatever happens I am sure it will be a memorable experience, if nothing else, and probably an incredible one as well. Good luck and I hope it goes well!