So you are stealing other people's food. Why is that? You are 29 years old, good age to get married and have oral or vaginal even anal sex with your own wife, why just the oral to other people's wife?
Anyway, we are not here as moral police, our job is to try help people. Answer to your question,
1. Yes its safe to have oral sex considering you or her, none of you have and infection or STD or some other transmitted disease which are non-STD. If not infected, then the other wont be infected too.
2. Nope, you cant find by yourself about all three mentioned. You need to go for thorough check up for her and yourself. The three category I am talking about are Infection - such as Urinary Track Infection, STD- start from HIV to Harpes. I am sure you have heard most of the names by now. Or non sex related yet transmitted like Hepatitis A, B, C etc Out of these second stack, some diseases has a clear symptom, you will know by a look, but the rest are not your job, thats why path labs gets paid for.
Dont consider STI/STD as only issue, there are more deadly non sex related by easily transmitted disease you can catch up.
3. Good that you heard, but I I haven't heard that oral sex can cause mouth/throat cancer. There are disease like mouth harpes, but not cancer, to my knowledge.
4. Yes you can perform with multiple partners, male female, any color, religion, marital status. Thats not the issue, the issue is those partners has to be STI/STD free as well as infection free and most important, the non sex related disease which can be transmitted through sex. How many would you go for test onto?
I am curious about one thing, for other people's wife you are so concerned about all these, what will happen when you get married? Would you go for oral or vaginal sex without any worry? Can you think of an issue that if you were having sex with other people's wife, your would be wife might have had sex with other women's husband too. Don't you think after getting married you should have a through check up for both of you? She wont say she had sex, just like you wont accept you ever had sex. Why this differentiation between other people's wife and your own wife? Both are same in a way, even after getting married to you, your wife might go for the same relationship just like you are planning with this married woman and more.