MilesBuzz! - newbie needs help planning flights to europe

So I have about a month in between jobs this summer and am planning to do some travelling around europe. I'm extremely flexible about dates/locations/times, but am tentatively thinking of flying into Riga to join a tour that will take me into Russia, Finland, and Sweden. The tour will then end in stockholm where I'd like to start doing some independent travel. I have a few questions about how I can optimize my flights that I was hoping to get some advice on.

1. I have access to about 90K in united miles and 30K of continental. However, I also have access to tons of american express membership reward miles and I'm trying to figure out whats the best use of miles. I'd like to book a flight into europe (ideally with a 1 - 2 stopover in london and maybe another stopover on the way back). Business / First class would be great, but is not essential. So far I've narrowed it down to some of the following options

Amex --> Continental --> Book Virgin to London and [?] to Riga?
Amex --> Virgin (take advantage of current 15% bonus) --> Virgin to London, how do I get to Riga?
Amex --> Air Canada --> Book swiss, lufthansa, or other star alliance
Amex --> ANA --> Book any star alliance
United --> Lufthansa or Swiss

Continental seems to be flexible with open jaw return as long as I come back to the same city. However its unclear to me how flexible these options are for building in a stopover.

On the way back to NY, I'd love to do a stopover in switzerland, though I'm open to other options. Are there limits on how long a stopover can be?

What about intra-europe travel. I was planning to save my miles and take advantage of the LCCs (but i'm realizing they aren't all that low cost) or look into train and/or bus options. Anybody have any experience or thoughts on this?

Finally - any other indepedent travellers in their 20s planning to be in europe this summer?

Thanks in advance for your help!

CO may allow you only a single stopover per overseas reward itinerary. At least that's what happened when I got CO reward travel for LHR-JRO/DAR-LHR openjaw on KL back in January. I did one stopover at AMS on the way over.
You might want to do your travel with two openjaw itineraries. One of them would be overseas. The second would be intraeurope. CO has partners like Aeroflot and KL. Depending on what you do, those may be good possibilities. The LCC options in Europe are beginning to sound like nothing but LCC options. LCC flight cancellations may really put you in a bind if the subsequent flight may not be for days.
Switzerland would be tricky. For what you want to do, OneWorld has the most possibilities in and out of Switzerland through British Air and Swiss International (London City airport becomes my favorite London entry point). Most of Europe is tiny, but between farout points like Riga, Switz & London, flying is easiest. These intraregional reward tickets can involve so few miles that I often get a round-trip reward and only use half of it for a oneway somewhere.
Most stopovers limit you to 30 days.

Amex --> Continental --> Book Virgin to London and [?] to Riga?
Amex --> Virgin (take advantage of current 15% bonus) --> Virgin to London, how do I get to Riga?
Amex --> Air Canada --> Book swiss, lufthansa, or other star alliance
Amex --> ANA --> Book any star alliance
United --> Lufthansa or Swiss



You will find that "fuel surcharges" on awards with Virgin, Air Canada, Lufthansa or Swiss will be much higher (several hundred dollars) than if you use CO or DL (your amex MR can also go to DL).

You can get to Riga using low cost carriers - use www.skyscanner.net Will show you airlines you probably never heard of before. Basically they are buses with wings, but get you to where you need to go (most of the time).

One thing to keep in mind - not only will you pay for luggage on LCC but - the weight limit can be as low as 15KG - 18KG.

Transatlantic flights allow 2 cases, each of 23KG - so keep in mind when you are packing IF you are going to use LCC in europe, try and stick to one case at 18KG max