What questions are inappropriate to ask a potential spouse?
Is it inappropriate for me to ask if he has committed zina for example? Particularly if the suitor is 30+ and raised in the West?
Female's in general don't have problems with this hence you must be the odd one out, there is no problem with the question but religiously the person can deny this even tough they partook.
If they admit to this, it's a massive red flag because your dealing with an irreligious shameless person you should steer far away from, hence it's a good test question to ascertain the character of the person you are dealing with.
In hindsight anyone above 30+ in the west, that looks healthy, is comfortable in their skin chances are that 80% of the time they did commit the deed and for those 20% that didn't that golden opportunity never came.
No one is immune to zina or has protection from this by default, companion Abu Hurraira used to seek protection from it daily, there is just barriers, some go out of their way to seek this out, others it takes a very long time with the right person to fall into this trap.
Can I request he undergoes STD testing prior to accepting a proposal if the answer is yes?
If a female asked me this question without giving her any reasons to suspect me, I would suspect her and show her the door, it's a shameless question to ask and if you ever feel the urge to ask this it means your either corrupt yourself, or your dealing with a person so corrupt you have genuine fears in which case you have another form of corruption deep within you to even be with such a person.
Also can I outright ask how much money he makes? (I would answer the same question if asked)
This is too direct and gives off the vibe that you are materialistic gold digger that only lives for this world, its the same as a man asking a women whether she would be "obedient to him", it's a red-flag because it shows your dealing with an authoritarian control freak, the same as the above.
There are much better ways to do this, you can simply ask him what field he works in, if it's public or private and for how many years? based on this you can easily find out roughly how much money he is making based on the market rate in the area for his field.
What questions should a Muslim man or woman ask?
The best question you can ask are the indirect probing ones were the person has no clue whatsoever the kind of information your trying to glean from them.
To give you an example to illustrate I wanted to know whether a women I just got to know has been circumcised or not (so I can rule her out quickly), I enquired about her trips to Somalia which is were such a things take place, I knew she took 2 trips at suspicious age range were this sort of thing happens and ruled her out based on this, there was no trips after this.
From the get go my biggest interest is their love for the "religion", their 'knowledge of it' followed by their character which is the easiest to judge, direct questions are awful because the person either gets annoyed or lies through their teeth to appease you.
If you must ask a question be very indifferent to it without taking a position the person could copy, I usually preface it " Why do so many people care about X, I couldn't careless about it myself", this forces the person to take a position without giving yours away.
Islamically how long can I delay nikah?
Ask a scholar.
My family would not be in favour of me marrying anyone I’ve known for less than a year. Particularly if my family does not know his?
How long is common?
You shouldn't care about time period, there are people you can get to know very well within a few weeks or months and those that take a considerable long time to get to know because they keep things to themselves.
My advice is to always select the person that is an open book, they come with their negatives as well but the up side is that you have a dacaad person you will get to know very quickly that's not scared to tell you the truth or be open to you about anything, these people can't hide anything.
I personally despise the opposite of these people because they tend to be the conniving, grudge baring, cunning, poisoned chalice Munafiq parasites, whenever I catch an individual who hesitates to share things or be open and honest (not talking about divulging private matters) I close the door on them very quick.
Figure out which personality trait you have and stick to your group, if your the latter type then avoid at all cost the first. This is one of the first things that I screen for