Can I use lag screws, lag bolts to hang swing vertically from a wood beam?
No, I would not trust those welds to hold for long under the loading effects due to swinging.

I would get bolts / hooks (3/8" dia min) that bolt through the plate which I would also have as 3/8" thick.

Edit, how about these:



Link to source https://www.easternjunglegym.com/heavy-duty-ductile-swing-hanger