Yes, you can perform Umrah while on a visit eVisa.
From the official eVisa website:
8.7 An eVisa is granted solely for tourist purposes or for the purpose of performing a umrah and it is not considered a visa for work. If you are granted an eVisa, you must not take up paid employment during your visit to KSA.
Yes, you can perform Umrah on a Saudi Arabian e-visa.
From this news article:
Among the most important features of the Saudi tourist visa, is the visa holder can benefit from it in performing Umrah, women need not be accompanied by a male relative, other than in the Haj season, and there is no need for a sponsor (kafeel) the way it is required for work ...
It's definitely not impolite to ask. In some countries (but not in Thailand I believe) it is considered impolite to say "no" so some people would think it is polite to lie and answer "yes" rather than "I don't know" or "no, it isn't halal", so you'd need to be careful.
But in many countries, including Thailand, sellers know that they have more potential ...