I did not miss the part of the blood, but since when does blood actually change your will? I don't see getting a blood transfusion changing who you are, I see it adding power, but not changing spirit.
Jesus was tempted, but he resisted, and in his temptation he never gave into the alleged promises, he did not summon any demons, cast any contagion spells, or likewise. Your example is a reason to say that this situation would not change the solar.
If you want to do it, do it, I am certainly not going to tell you you can't. I have added every template to ever creature in existence practically, although I must admit I have never made a half fiend celestial before.
I just think the result wouldn't be as uncertain as you are looking for. Basically, I think you have three results
1) Solar loses all celestial powers and becomes a fiend using fiendish powers and openly embraces what they have become.
2) Solar loses some celestial powers, gains some fiendish powers but chooses not to use the fiendish powers and is thereby a weak solar.
3) Solar resists the effects does not lose any solar powers and does not gain any fiendish powers, and is just a severly tortured solar.
Personally I think the actual result would be 3. I think 2 wouldn't end up being very usable because of the diminshed capactity and inner conflict. 1 isn't anything special, a fiend that looks like a solar.
You can add the template to the solar, no problem, that doesn't mean he would use any of the abilities gained from it. Movies and novels are full of heroes who resist the evil influences that have been places upon them. Off the top of my head, Blade born a vampire, but resisting the thirst. Conan the barbarian, grows up as a slave and a pit fighter, but regardless of how he was treated, or what was instilled in him by his surroundings, in the end stays on a path of determined justice and is a hero. You could use the Terminator from Terminator 2 if you like, you see him perfectly capable of destroying anything he comes across, but when John tells him to stop killng people he does, although he could, he resists. I think the solar is pretty much the same, just because you recieve a blood transfusion and get tortured a bit doesn't have to make you a monster.