I'm considering getting a new flute. I definitely want an offset G, but would prefer not to get the split E mechanism. Looking at the Yamaha line, I notice that this is NOT a combination that they offer, except on the 400 line. Am I right in thinking that you can't get offset G WITHOUT the split E on the 500-700 series? Any ideas why this might be?

Yamaha does their own thing. Just the way it is.

What are you looking for specifically and what level is your playing at? Also, what is your budget? There are many other choices besides Yamaha, but we need to know a bit more information to narrow it down.

I'm an adult beginner, and have been playing/taking lessons for a year and change. I currently play a Yamaha 225N. I had an opportunity to play two other flutes last week - an old Haynes, and an old Powell, and was interested to see how they cleared up some problems which I had assumed were my technique. I also had a chance to have a professional flautist spend some time on my Yamaha. She noted that it was a bit inconsistent - sometimes it played fine, sometimes it gave trouble.

Budgetwise, I'm thinking of spending between $2.5k and $3.5K. My guess is that this puts me at a flute which is better than I really need at this point, but I'd rather get something that will last me a while (or forever), instead of needing to upgrade again in a couple of years.

The Powell I played had open holes, which didn't cause me much trouble, so I'm thinking of going that way. I'm also thinking of a B foot, and definitely want an offset G. I've been warned away from the split E mechanism from a number of sources. (My third octave E can be spotty, but it played like a dream on the Powell). The one issue I had with the Powell was that the sound was a bit too bright for my taste.

I'm currently thinking about the following options:
Sankyo CF-201
Muramatsu EX
Resona R200-O

I would greatly appreciate any thoughts you have to share.

I recommend the Muramatsu EXIII. Excelent for the price, that must be around $3500.00. You can try directly with Muramatsu America (Mrs.Valerie), or you may ask Fluteworld send some models for you to try. They also have many other brands.
Besides my preference (which is because I actually have one as a backup flute), it's always better if you can experiment playing before you buy.

Try a Muramatsu-- they play beautifully and are definitely worth the cost.

sometimes it played fine, sometimes it gave trouble.

That indicates that the flute needs some attention from a repairer. You might find that for a small outlay the sound & reliability will improve greatly & you might be happy with that for a bit longer.
Many times I have spent an hour or less checking out instruments that owners, who have upgraded, wish to sell on & I get comments to the effect that they now play as well as the new one!

But don't let me put you off buying a new one - there have been some good suggestions about possibilities in your price range. The message I get from many players & teachers is that the higher models of Yamaha don't really match up to other makers for a similar price despite their 200 range being the best of the student flutes.

Good luck & happy playing.