A longer way to say that would be "Ĉu estas tio, kion vi skribis?" These t-,k- pairs are pretty common among all the correlatives. The correlatives are one of the harder parts of grammar to get down because it's just rote memorization of -o/-u/-e, etc. but once you do have them down, they're very useful
Can I pronounce the "t" and "d" dental?
Sure, if you want. The point is to avoid aspirating them, so if doing them dentally works best for you, go for it
Can I pronounce the "r" as [ɾ]? Well, except for at the beginning of words, where I'd pronounce it normally (/r/).
Yeah, I actually do the same thing you're describing here.
I notice some of the speakers on Lernu! use /ʎ/ pretty much randomly throughout the recordings. That's just a non-native accent, right?
I remember not being too pleased with the recordings on lernu. You can definitely tell in some of them "oh this person's from Poland" or "oh this person's German." I wrote a little rant about that in the Esperanto Lingua Franca thread
Are "sc" and "kn" pronounced as [sts] and [kn] or some other way?
Nope, you got it right. "sc" is very often mispronounced as a long "s", but it should be as you wrote it.
And for the record, I did Ana Pana on lernu to get me going, then used Teach Yourself Esperanto since I'm not too big on web-based learning. It's one of the few TY books that I really like.