1. Paris, Fr to Caen, Fr - March 15, 2022 08:29-10:46 (4 seats first class)
   This is a regional train, which cannot be reserved online. It can be done at a ticket office in France. However, according to this page, reservation is only needed on Friday/Saturday/Sunday and holidays. 15 March is a Tuesday.
2. Paris, Fr to Brussels - March 18, 2022 16:46-18:43 (also 4 seats 1st class?)
   This is a journey with a change in Lille between 2 TGV trains, which makes it slower and quite expensive.
   Why not book a direct train from Paris at 17:55 for 25 EUR p.p.? It seems no 1st class is available anymore but you'll survive in 2nd class for the 1h and 22 minutes.
   You can also book at a French ticket office.

The second reservation should also be bookable via Eurail (with 2 EUR booking fee per person). What do you mean by "won't let me through”? If there's an error message, please post a screenshot. If not, please post a screenshot of where you get stuck.

Thank you for your response.  When I go to the rail system direct for the Paris/Brussels connection, even the time that you suggest,, there are plenty of 1st class tickets available.  Is it just because we are trying to use a rail pass?  

Thank you.

I'm not sure which website you mean by "rail system direct” but the fact is that there's a quota for pass holder seats in the direct Paris - Brussels trains. That means they sell out earlier than normal tickets.