I really don't think this is the best way to think about things. Parallel fifths (or other contrapuntal "errors") can be fine. In some contexts, they're very effective! In others, they are deeply enough buried that they don't really strike the ear. Open a score by, say, Handel, and with a little bit of searching, you will likely be able to find parallel fourths, fifths, or octaves somewhere.
You shouldn't think, "Oh, this piece sounds good, but it has the technical error of parallel fifths here." You should think, "Maybe this piece would sound better if I changed it to avoid the parallel fifths here." And if you find that it doesn't sound better, leave the "error" there.
Traditionally, two timpani were used, with a combined range of F-F. There are bigger and smaller drums that extend the range downward and upward, so you may be OK going down to the low D. The bigger issue, I think, is to decide how many drums your score is going to call for and then stick to that. In Beethoven's day and earlier, two drums were used, usually tuned to the tonic and dominant of whatever key the piece is in. In modern orchestras, it's common to have four drums. The usual practice is to decide what pitches those drums are each tuned to and indicate it at the beginning of the score, then only write those notes for the timpani. If you need a different note, the drums can be retuned in the middle of the piece, given a few bars of rests - just add a note to the score at the point when the tuning should be altered. You can also do glissandi where the drum is retuned while a roll is being played, or after a note has been struck. Now, maybe an expert timpanist could play something like what you have in m. 8 on just two or four drums by retuning while playing, but you'd still inevitably get a pitch-bending effect, which I don't think is what you want.
In general, though, the exact pitch that the timpani is playing - especially in loud passages like this - is less important than you might think. If you were, for instance, to rewrite that measure with the timpani playing the same rhythm but holding to the initial pitch of C, it would probably be just as effective. The combination of the timpani with the cellos and basses often tricks the ear into hearing a single bass line, rather than hearing the timpani in harmony with the bass line. Or, if you decide to use four drums, you could have them tuned D-F-A flat-C, and either remove the sixteenth-note passing tones from the downward figure in m. 8 or move those passing tones up or down a step to put them on playable notes.