Legal concern about distributing models on different platforms.

It depends. You have to check conditions on every store where you upload your assets. Generally, almost every store allows artists to sell non exclusively and you're free to sell your assets on as many stores as you like. However if you choose to sell exclusively in one of the stores, you can't sell that asset anywhere else. And yes, you can set different prices for the same item on different stores, although there are stores, that won't let you set your price at all - instead they do it for you.

As for mixing distribution between monetized and free for the same asset, i think you are free to do that as well, but i wouldn't recomend it, because free and monetized models usually are shared under different licence and it would be hard to you to proove breach of licencing agreement if such need would arise in the future.

I can say for unity unreal and cgtrader, that they are non exclusive. and you decide the price strategy. i work hard, i need every penny, so it is up to one to study how it works out. i have individual items here (just started) and bundles elsewhere. i can not recommed sites like 3DOcean. They price items way they want, they give 33% only for maker as non exclusive. CGTrader submission process is a swift so I like the site, but I will see how game items will sell - site has been profiled as High Poly and even the low poly competitions are 50k. i'd say low poly items are 200-2000 triangles.