I don't know why your posts are delayed or don't appear. You might think about writing in Word so that you could save it in your Documents. Then you could copy & paste, if needed.
Denver, I guarantee you that every word you say to FIL will get back to your W. There is the SIL, MIL, & FIL who are talking to each other and who want to see you and W get back together. I know how these things work. He will tell his W and she will tell the sister and sister will tell your W. Bank on it. So, be careful what you say. I would suggest that you only say what you would say to W's face. Don't give away your plans.
Your FIL means well, but he is applying pressure b/c he is concerned that your W will succumb to OM. I would not advise you to send a rose everyday and then a dozen or more on Valentine's. It's too soon and she has not said anything that implies that she is considering reconciling (unless I missed it). She has opened a window, maybe, to be friendly. Your biggest mistake right now would be to start romantic over-kill.
Your goal was for her to miss you, right? Well, she is missing you or she would not have these extremely long conversations. So, that is great!
As it's already been pointed out...this is a fragile state. This is the state that a LBH can mess up b/c he's ready to get the M back to normal again. But, she can't be rushed like that. She needs to pursue you a little longer. She is interested. That is a good sign. You are responding well. The fine line is to know how to respond without going too far or jumping to the next level before she's ready. That is what your FIL is trying to get you to do, and I think it would mess things up.
She has her eye on you and OM is not so interesting to her now. But, she needs to get him out of her system once and for all and know that you are the only one for her. You cannot afford to let her get you back so easily. So, be friendly, listen and validate, but don't pursue. Wait just a little longer. Wait and see if she hints at the two of you going out to eat, or going to see SS doing something. (She may use him as her ticket). That will be your cue. Remember, do not over-kill. This is the biggest thing for WAW's. She needs to do some work to get you back. And, your part of the work is to continue DBing and go slowly.
By the week before Valentine's Day rolls around (I think it falls on a Sunday this year), you should see more signs from her.....one way or the other. Do not send a huge bouquet of flowers. Instead, the Tuesday before Valentine's, have the florist deliver one long stem rose with a written invitation to have dinner with you Valentine's night. If she has other plans or turns you down.....well then you'll know she is not ready.
It is just two weeks away. Be friendly when she contacts you. If she accepts a dinner date Valentine's, then she is ready for you to slowly romance her. I said, sloooooowly.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!