As the only professional team it's even more important that they aren't filled with non-Frenchman.
I agree re them having to stay in SL which is why I'd make them exempt from relegation. I think it's daft that you have effectively a development team but make them have such conflicting priorities.

Development is a noble ideal but I don't think the French public would turn out to see their team get walloped every week. I doubt there are sufficient French players of sufficient quality to prevent this at present. Which is one reason why development in France - and over here too - needs to be from the grass roots up. The notion that the top level of the domestic game should be used to develop below-par players into good ones en masse doesn't work for me. I can't think of any sport with a spectator presence that uses such a model for the simple reason that the customer perceives it for what it is - second rate. In no time we're back in the franchise situation, where large rafts of our fan base perceive the competition as meaningless.

I really meant development as in developing the French national team and the sport as a whole in France.

Nor do I expect their team to contain only French players. There's nothing wrong with adding quality overseas players to your squad.
But they should be an addition, not the bulk and mainstay of your team.
I may have got the numbers slightly wrong but IIRC if Catalans sign Willie Mason they could put out a starting lineup that doesn't include any French players at all.

I don't expect Catalans to not use overseas players but I do expect them to not have too many in their squad. And I think most of us would agree that 13 is too many (or 11 if you take out the 2 English players). In the same way as I thought when Hull KR or London had 10+ overseas players it was too many.
Aside from it meaning they develop fewer players, I also don't think it helps a club engage with its support base as much.

Shouldn't they just be restricted to the same number of overseas signings as the rest of SL?

Players only get better by playing. Ok, some of the French guys might struggle early on but there is enough decent French players coupled with a full quota of overseas players to form a competitive 13/15 players.

Injuries would be problematic but most teams outside the top few have that issue already.

I assumed they were subject to the same rules.
Obviously the 2 English players don't count in the same way as French don't count on English clubs quotas. But I still don't know how they're getting the rest in under the NFT and Overseas Quotas.