I believe this melody well conforms with the harmony, and does not offense my ear and the rules about counterpoint and chord progression. However, when trying to fill in the inner part, I realize the 4-th degree in IV6 will either go to 5, which forms a parallel fifth with soprano, or go to 7, an awkward tritone leap. If going to 2, the fifth of V will be doubled (soprano already has a 2), which is incorrect as V is in root position. As a result, some exception of rules must be made. But which?
Changing the doubling of IV6 will not help, as the issue concerns the progression of the root, which must appear in at least one inner voice. The doubling is definitely the lowest-priority rule, so 4 should proceed to 2, possibly jumping up to 5 if necessary to connect to the next harmony. (Taking advantage of the fact that the original asker did not specify a key, I note that G major yields comfortable ranges.)
B - G - A G - G - F# D - C - A -(D) G - E - D
Alternatively, if only the bass and soprano are fixed, the progression I-vi-V yields nice root doublings:
B - G - A D - E - F# G - B - D G - E - D
The convention is "Roots a second apart":
"Roots a Second Apart (no common tone): Move the upper three voices in contrary motion to the bass, making sure each voice moves to the nearest chord tone of the next chord; the roots of both chords should be doubled."
Though the first chord is an inversion:
"When one of the two triads is in inversion, write to or from the doubled note first, using oblique or contrary motion if possible, and then fill in the remaining voices."
I am working a picture of the score for your situation, but hopefully this will help
A. I see your problem, even with doubling the tonic in IV6
B. If you are able, going to a V7 would help avoid issues, but I understand if you can't/don't want to
C. This is risky, but you could go in similar motion with the bass, doubling the fifth of V consequently
D. I guess if you doubled the third of IV6 you could escape, but I completely understand if that is not acceptable able for you. Plus it kinda sounds terrible. The tenors have that tritone you were trying to avoid, but they're hopping to "ti," so it's not that bad.
I believe the best option is for the alto to go to B (going to "ti" from anywhere shouldn't be hard) and make the tenor go to high G (sorry fellow tenors). Or you could consider my options above or something else. Who knows?
I apologize for any misunderstandings and/or errors. I am no expert. By any means.