Rebbe,
On daf zayin (amud alef d"h B'levaer), Tosfos indicates that if one makes a beracha before a mitzvah is done if it's the one doing the mitzvah he makes the beracha before the action. When there is a chasuna and someone other than the chosson is making the birchos erusin he makes the beracha after the ring is given.
In our days when we always have a mesader kiddushin, we don't do that. Our practice is that the Rav makes 2 berachos and then the chosson says harey at etc. and gives the kallah the ring.
(a) Did that change since Tosfos days or is it a machlokes? Also was there a time when there was no mesader kiddushin?
(b) In addition, there are certain mitzvos such as netillas yadayim which we make the beracha after the action. Why is that? Women also make the beracha while immersed in the mikva. Is this a practical matter because a person shouldn't make a beracha while undressed or is it for another reason.
B'kavod,
Sam Kosofsky
(a) It is a Machlokes. The Shulchan Aruch (34:1) Paskens that we should do it the way it is presently done. The Acharonim (see Chelkas Mechokek and Beis Shmuel) explain that the Shulchan Aruch follows the opinion of those who hold that this is necessary as it is Over l'Asiyasan. It appears that in the time of the Gemara there was also a Mesader Kedushin, as implied by the Gemara in Kedushin (6a) that one should not deal with someone who is not a Baki in Gitin and Kedushin. [There is a discussion amongst the Poskim whether one can honor a regular person with Siddur Kedushin. For a disucssion on the subject at length, see Mevakshei Torah on Kedushin v'Nisuin.]
(b) It is possible that the Berachah is said after Netilas Yadayim as one is able to better recite the Berachah afterwards, as he now has clean hands (this is especially true when waking up in the morning). There is an entire Siman in Shulchan Aruch (Y.D. 200) which deals with your question about the Berachah said by women in the Mikveh (Ayen Sham).
All the best,
Yaakov Montrose