Welcome to the Forum. I'll be happy to comment.  The exposures you describe were low risk and only your receipt of oral sex places you at any risk for STI and what risk there is is small. You are correct to not be concerned about HIV.  As you point out, the STIs you are most at risk for at gonorrhea and non-chlamydial NGU.  If you had gotten gonorrhea there is at least a 95% chance you would have developed obvious urethral discharge and burning on urination.  The slight irritation that you describe could be NGU although the onset is a bit late and the absence of visible discharge likewise reduces the possibility that this is NGU although the slight staining that you notice on your underwear does suggest there may be a slight discharge.  At this point, the single best test for sorting this our would be to seek a urethral gram stain in which penile secretions are obtained with a small swab and looked at under the microscope.  The test is best performed when the specimen is collected at a time at least an hour after you have last urinated as urination tends to "flush out" the white blood cells if present.   If there are increased numbers of white blood cells present, this is compatible with NGU and probably warrants treatment. 

thus, to specifically address your questions:

1. See above.  chlamydia is VERY rare after oral sex, non-chlamydial NGU however is not all that uncommon. The late onset of your symptoms makes it less likely that you have NGU however.  I would add that your suggestion that your guilt and perhaps anxiety over your exposure may also place you in a state of heightened awareness in which what you are noticing are normal sensations .  A urethral Gram stain performed at an STD or sexual health clinic would help to sort this out. 

2.  See above.  A urethral gram stain would be the best test.  Urine testing for gonorrhea and chlamydia is reasonable but I anticipate the results would be negative.

3.  Even if you have non-chlamydial NGU you are probably not putting your wife at risk.  In the UK many specialists do not even suggest treating partners of men who have non-chlamydial NGU.

I hope these comments are helpful.  Please feel free to follow-up if any of this is unclear.  EWH