It might be an idea to just replant that hc stem by stem and it'll carpet fine. To be honest, almost anything is possible with the tank, iwagumi's I don't think are all that easy to maintain compared to say a wood scape with slow growers and lower levels of light. Hairgrass would be about the same as blyxa I guess, they're both very fast, I think blyxa might be easier to maintain. Diatoms are normally cleaned up by ottos better than anything else. I tend to add about 6-8 in a tank this sort of size. Other than that rather predictably I'd point you to James c's guide.