Open Jaws and Thalys Train to Amsterdam

(1) Use a search engine like kayak.com or google flights to find the flights that work best for you. Search using "multi-city" rather than "round trip." Then book your flights on the website of the airline you choose, again using "multi-city."

(2) There's no right or wrong answer to this question, it depends on what flights are most convenient or least costly, what time of year, and your energy level. You'll have at least one day dealing with jetlag and sleep deprivation, maybe more. You don't want that to be the day your tour starts. So you may want to start with Amsterdam for that reason. But if you can spend more time in Europe than just the Amsterdam days and the tour days, then you have more flexibility. EDIT -- Many on this forum would agree that AMS is generally an easier airport than CDG, and this may affect your choice. But with open-jaw you get both airports anyway.

(3) We took the Thalys from Amsterdam to Paris in 2013 and it was great. Second class was fine, although we found ourselves in seats facing back instead of forward, which I found uncomfortable. But this wasn't a function of the class we chose, and the seats themselves were fine. As our host reminds us, the second class passengers leave and arrive at exactly the same time as those in first. We bought our non-refundable tickets a few months ahead of the trip, as I recall.

Your Thalys questions will probably be answered here:
http://seat61.com/international-trains/trains-from-Paris.htm#Paris-Amsterdam
My Thalys trip was quite a while ago, but similar to other high-speed trains I have taken.

Open jaw is simple to arrange. Most airlines call it multi-trip. You will want to explore different itineraries to see what works out best. Prices may vary depending on the order.

Either way will work. The Thalys train is very easy in either direction. I took it in April just Paris to Brussels and preferred 1st just because the seat configuration is a line of single seats down one side which worked for me as a solo traveler plus fewer seats in the car. I felt less exposed to Covid.

Download the Thalys app so your tickets are accessible there.

April will be tulip time so you could work in Keukenhof Gardens. Be mindful of Easter - not that things are closed but that it’s spring holidays for schools so an increase in crowds. Also be mindful of Kings Day which I think is April 27 and is a huge holiday.

There is going to be a Vermeer exhibition at the Rijksmuseum from Feb to June. If his art is of interest, you might consider signing up for notification of when tickets go on sale.

https://www.rijksmuseum.nl/en/whats-on/exhibitions/vermeer

If you do Amsterdam first, I recommend you get to Paris at least one day before the tour start. There can sometimes be train strikes so give yourself some flex time.

We took the Thalys from Paris to Amsterdam and it was excellent. I booked as far in advance as I could to get the discount. It is a number of years ago, so I don't remember which class, but as I had 3 teens with me I doubt it was first! Very clean, comfortable and fast.
I would personally do Amsterdam at the end. We have done that twice as it is one of the few cities we can get direct flights home from. I find the Netherlands a great place to finish the trip. Also as it is April, it may be a bit warmer the further into the month.

It sounds maybe like you don't have much experience in European train travel. You may wish to do a bit of general research, so you'll feel more comfortable:
https://www.ricksteves.com/travel-tips/transportation/trains
https://www.seat61.com/

I buy my train tickets through the train provider's own website, and print out bar coded tickets before we leave. Note that April Thalys tickets are probably not yet available.

As another noted, the key is what cities are easy and affordable to get to from your home area. In fact, you can spend a whole vacation daytripping from Amsterdam, especially in tulip season. I personally find a 3.5 hour trip tedious, but it certainly depends on how often you go to Europe for a vacation.

I don't see the difference between before and after the Rick Steves tour, but it is a good rule to allow for at least one-day early arrival for any scheduled tour or cruise, from any provider. In the case of this particular tour, you could catch up if they left without you, I think.

If you have used the Search box, top center, you may already know that airline sites use the term "Multi-City Journey" for this, not "Open-Jaw", which is an old travel-agent term. Have you ever bought an international airline ticket before? I only mention this because there's a knack to reading the list of trips the website will offer you. So you have to learn how to screen out (if you wish) trips with a stop or change, or how to rank the departures by price instead of by time of day.

It's an advanced topic you may choose to neglect, but many people (myself included) believe that sophisticated airlines (like United, one I use a lot) look up your search history and are likely to create higher prices if you repeatedly search the same destination on multiple days in the same month. I try to at least search and buy on different computers, but I think they would both show the same IP address from inside my home. (Verizon FIOS Internet.)

You might enjoy watching some of the Thalys trip reports available on You Tube. There are some very good ones are done by Paul Lucas and Noel Phillips. These often show the entire process starting at checkin and boarding, as well as giving hints about travel class selection.

Tim gives lots of good advice. I am less leery about search history manipulations. Frequently clearing my browser history is as much as I do. Does it work? Who knows. I do think that your freelance exploration will be more satisfactory after the tour because you will be in the rhythm of tourism.