Yes, the womyn are truthful. Given they are the sultan's harem on orders to do so, the "or else" is likely to be painful.
If the suitor wants, he can talk say "sorry love" 99 times and hope that the 100th has the largest dowry. Of course, that means a 99% chance of ending up dead, but the company is excellent until then, however brief the encounters may be. If he says "yes" to someone before the 100th wife, the situation changes so that when the rest will parade by, he will not be hoping to hear of a higher dowry after that but will be praying each time that he will hear "I can't marry you, my dowry is not the largest yet" rather than "honored guest, my dowry is the largest yet..wha?? you've chosen already? GUARDS! Execute this cretin for so insulting me and through me the sultan!"
You are correct on both counts. If the guest can see all 100 and receive the yes/no information on all of them before choosing he has a near 100% chance (near, given margin for human error such as what was her name again, or I know that Jasmine is the right answer but I just could not say anything but Orchid as she is so lovely that she took my heart with the first glance and is worth dying for in a hopeless romantic gesture) of ending up married, rich, and alive. If not, which is what I intended, the chance is much less than 100%, though the 37% chance is still imo a lot higher than one might expect on first glance.