To answer your first question: If you compare the regular prices to each other, you will see that a one-way ticket Strasbourg-Paris is 93€ while a one-way ticket Karlsruhe-Paris costs 114€, so as expected the longer trip costs more. It seems to me that the ticket prices you quote are for discount tickets (which at SNCF you can find in the first and the second column):
These discount tickets are only available on some (and not an all) routes and their availability is limited. They are sold on a first-come-first-served basis, so either there are no discount tickets for most trains from Strasbourg to Paris or they are already sold out (at least the cheapest ones). Also there availability might be different between SNCF and DB.
Luckily for you, it seems SNCF allows you to board the train in Strasbourg when you have a ticket Karlsruhe-Paris. This is supported by a certified answer on Questions SNCF.
Note though that according to above link if you have a seat reservation and you don't take your seat at the origin of the reservation, the conductor might give your seat to somebody else. I'm not sure whether you can buy a separate seat reservation with SNCF but SNCF only sells a limited number of tickets without assigned seat, so you might find a seat anyway.
Please note though that different rules might apply to trains in international traffic between Germany and France. I couldn't find anything though limiting above answer to only national trains.