I've noticed a trend where the regular present tense is not accepted for many of the English sentences in this skill- for example here I wrote "Grandma stands and does the dishes", which means basically the same thing but it was marked incorrect. Does Finnish have a similar difference between [verb]ing and just [verb] in the present tense that hasn't been taught yet? Or were all of these translations overlooked?
Present tense should be always accepted. We don't have the same kind of continuous tense like English does. Our kind of continuous tense would say mummo on seisomassa ja tiskaamassa but that's just funny. However, just saying mummo on tiskaamassa is just fine and means "grandma is doing the dishes (as we speak)". If I had to specify that she's doing it while standing, I'd say mummo on tiskaamassa seisten.
If you're talking about a contextual translation from Finnish to English, yes, the use of the proper person of the verb "to be" should be used as an auxiliary verb, to indicate that someone is in the process of doing something. Grandma stands and does (whatever) would tend to convey a general practice, something she usually does, rather than something that she's in the process of doing it.