I'd recommend using the interail rail planner app, here is what it says about ferries from Stockholm to Helsinki (via Mariehamn)
Tallink Silja Oy
Offer: Enjoy a 20-40% discount on online bookings for Tallinnk Silja Oy ferries operating the following routes to and from Sweden:
Helsinki-Stockholm via Mariehamn
...
Info: To make a booking please go to www.tallinksilja.com. To make contact by telephone, please use one of the following numbers: Finland +358 (0)600 15700 or Sweden +46 (0)822 2140. Offer code: EURAIL
Please note: The discount varies depending on departure date and booking situation. The discount does not apply to port taxes.
Link for the app: https://play.google.com/store/apps/details?id=de.hafas.android.eurail
I suggest you to use a handy app: Rail Planner. You input starting and end point to get your path. It's 100% offline, so you can use it even when you're here in Europe without being robbed by telcos. Also, it has options to avoid trains with compulsory reservation so, if you're in a hurry and haven't booked in advance, you can get on a regional train with your eurail ticket and nothing else.
It would be smart to use night trains but those often needs reservation, especially long routes. You might also be forced to pay for bunk beds since some night trains simply do not have seats (if I'm not mistaken this info is available through that app).
Bunk beds are often WAY more expensive than a cheap hostel. For example I almost had to pay 70€ to go from Munich to Verona. Being a cheap bastard I travelled through central Europe for 30 hours straight with a total expense of 5€, instead. Indeed it was quite an adventure ;)
By the way, going from Gothenburg in Sweden to Paris in 7 days means you won't get off the train, IMHO.
Download the DB Navigator app (here is the link if you're using Android: https://play.google.com/store/apps/details?id=de.hafas.android.db). Search for connections from Stuttgart to Berlin. Tap "Options" -> "Means of transportation" and uncheck the ones for which the 9€ ticket is not valid.
Good luck!
Search for the connections in the railplanner app. If the route is in the app, it is good for eurail (until explicitly marked otherwise). If it is not in the app, the pass may or may not be valid.
Also in the app: tap "more" -> "rail network guides". There is a comprehensive list of operators accepting the pass.
I'm not sure where you are looking then
For most of the journeys tickets are not available via this website but I'm pretty sure you can book them via the ÖBB website.
My opinion (which is worth what you just paid for it - zero) is yes, European travel will be on for summer 2022. My plan is for June (or September) and it can't come soon enough!
Get you passes from somewhere that offers refunds as early as you like (phone them and check their refund policies).
I have learned the hard way to only book accommodation on a 'free cancellation' basis through aggregators such as booking.com or hotels.com (others are available) - as much as I hate their business model. I will book mine about 6 moths before travel. I need to book accommodation in advance due to disability requirements, many people like to be more flexible.
250 euro per person yes.
For example this route costs around 150 euro: https://www.google.com/flights/#flt=/m/0k3p.KTW.2017-07-02.AMSKTW0HV5563*KTW.DSA.2017-07-06.KTWDSA0W61015*DSA.SOF.2017-07-10.DSASOF0W64316*SOF.BUD.2017-07-14.SOFBUD0W62472*BUD.EIN.2017-07-20.BUDEIN0W62271;c:EUR;e:1;sd:1;t:f;tt:m;sp:.EUR.14700*.EUR...
Always go with 'less is more' in terms of size, but there's a lot of variation and options available depending on your exact requirements.
What I did for a 5 week trip was get one large bag for planes/trains/etc, which I left in hostels when I was sightseeing, and a much much smaller bag for camera/raincoat/book/water.
In my case I got a Osprey Porter 46 for the big bag: https://www.ospreyeurope.com/shop/gb_en/porter-46?store=gb_en
and a lowepro streamline sling for the day bag: https://www.amazon.com/Streamline-Camera-Sling-Bag-Lowepro/dp/B00DEH5XPS
Edit: for that specific bag you linked, the capacity looks good but it doesn't look very comfortable to me, and you may be wearing it for long periods when travelling.
Strongly consider getting a small Daypack too for when you're out and about
http://www.amazon.co.uk/dp/B00BNTOW10/ref=pe_385721_51767431_TE_dp_1 is the rucksack, and I didn't book from Vienna to Budapest and it was fine. Not sure about Paris but you can always book on the day of travel or the night before if you are worried and it really doesn't break the bank!
Including absolutely everything, just over £1000 for me, but I've got a feeling trans-atlantic flights might make that a bit hard for you ;) couple more things, we had this guide book which had loads of useful information in it, and we were going to get the full train timetable, but left it too late to order it.
Not affiliated but I like the rail planner app (https://play.google.com/store/apps/details?id=de.hafas.android.eurail&hl=en) which works fully offline and has an option in the route planner for trains without a compulsory reservation. I wouldn't risk it at the station if you already know the trains to be full. If you don't want/arnt able to download it post your routes and I'll get back to you.