I seriously doubt this is caused by masturbation unless you are doing penetration with a dildo that is not a quality silicon rubber.

As long as his "rash" is not painful and your vagina is not causing you any major discomfort, I wouldn't worry.

I feel any cold pressed oil like grape seed, apricot, avocado are all good to use as lubricants.

BV is a broad category that includes yeast overgrowths, the most common problem for modern vaginas due to the amount of sugar consumed.

Yes it is possible to have an allergic response to a man's semen.

Yes, you can get OTC remedies for this.