I am unable to answer your questions about the staining of Kleihauer slides, as this most definitely NOT my area of expertise, but I will have a crack at the bonus question (but with a comment about terminology thrown in!).
Yes, a transfusion certainly will have implications on the FMH screening/quantification, and it will lower the results, but only if the transfusion is given for an ante-partum haemorrhage very close to birth, or a post-partum haemorrhage soon after birth, but before the sample has been taken to estimate the FMH.
If you think about it, the only reason for the woman to have a transfusion at these times is if she, herself, has had a haemorrhage. The ratio of "adult" red cells to foetal red cells she loses will be identical to the ration in her circulation, but as soon as she receives a transfusion, the ratio of "adult" red cells to foetal red cells in her circulation will rise. The reason for this is that those red cells being transfused to her would contain no foetal red cells, and so, in effect, these transfused "adult" red cells will "dilute" the mixture of maternally-derived "adult" red cells and foetal red cells in her circulation. Of course, and also in effect, this means that the amount of "adult" red cells will be concentrated (maternal red cells + transfused red cells), meaning that the ratio of foetal red cells to "adult" red cells in the circulation will be lower (if you like, it is a bit like the more saline you add to an antibody, the weaker will be the reaction, as is the case with an antibody titration).
Now, it may be argued that the ratio of foetal red cells to maternal red cells that are bled out during the haemorrhage will be the same as that of the original blood in the maternal circulation, which means that the actual volume of foetal red cells in the maternal circulation (as opposed to on the Labour Ward floor) is reduced, and I would find it difficult to gainsay this, so it may well be that the calculated amount of anti-D immunoglobulin required to prevent sensitisation to the D antigen, and so the dilution of the foetal red cells remaining in the maternal circulation, versus the actual volume of the foetal red cells in the circulation is irrelevant, and will still be adequate, but I would err on the side of caution, and give a slightly higher dose of anti-D immunoglobulin.
Now for my comment!
There is no such thing, and never has been such a thing as the Rhesus Blood Group System. Rhesus with an upper case "R" was an ancient King of Thrace. rhesus with a lower case "r" is a monkey. As I have said many times, neither Rhesus, nor rhesus worked (directly) in blood transfusion.
The correct terminology for the Blood Group System is Rh. However, the gene is RHD, the protein carrying the red cell antigen is RhD, but the antigen itself is just plain D (not Rhesus D, not rhesus D and not Rh D), and so the pregnant women of which you are talking are D Negative.
The wrong name has come about because Landsteiner and Wiener injected rhesus monkey red cells into various animals (rabbits and guinea pigs) in an effort to discover more antibodies against antigens expressed on human red cells. They managed to do so, and the antibody they found was thought to be identical to the anti-D described in a human by Levine and Stetson, but in the early 1960's it was found that the antibody described by Landsteiner and Wiener, and that described by Levine and Stetson were far from identical (and, indeed, the genes encoding the antigens are found on different chromosomes). As a result, the antibody described by Landsteiner and Wiener was designated as anti-LW, and this particular Blood Group System is now designated as LW.