I’m confused with your math. If the Passover is slain/eaten on the 14th/15th, and then you’re trying to make a connection to a Feast that wasn’t even instituted until Israel crossed over into the land, I’m not sure your calculations are correct. Don’t get me wrong, I see that the oath (marriage) at Sinai can certainly be what the Feast of Shavuot was commemorating but to explain it with your calculations doesn’t work.

15th of 1st month is when Passover is eaten (Slaying the Passover lamb is the only celebrated action while wandering in the wilderness and done only twice. First in Egypt and second time a year later. No Feasts were celebrated)

If that is day 1, then to start a count of 50 days, you would have to wait until the day after the weekly Sabbath. Not knowing when that is, how can you begin a count? If we assume that day 2 is the weekly Sabbath, then we would start the count on the next day. So at the very least, we are on day 17 (to begin our 50 day count) of the first month. And 29 or 30 days contained in a month would only be 12 or 13 days later.

Add another 29/30 days for the second month plus 1 day for the first day of the third month and you are somewhere around 41-43 days into the 50 day countdown. As well, scripture telling us that Moses went up and down to meet with YHVH a few times and it is the third day that the people will meet with God, tells me that going up and coming down is not a long trip and all done on the same day. It also tells me that it’s not three days later. When YHVH will meet with the people. YHVH counts the day they are on as the first day. In other words when He says “today, tomorrow and third day”, He is counting only two days from the day they are on. Therefore, we are at most only 45-46 days into a 50 day countdown.

While many may think I’m straining gnats, the point I am trying to make is that the Feast of Shavuot is not celebrated outside of the land proven by the fact that they did not celebrate any Feast nor did they slay another Passover lamb after they left Sinai and after the first anniversary, and not until they crossed over into the land as Joshua 5 states. Feasts were not celebrated in the wilderness or at the base of Mt. Sinai. I see that in theory there is a connection in the marriage covenant and Feast of Shavuot but I cannot see that the math supports the Feast day instructions.

All of this to say that many people use the wilderness time to validate ignoring the Feast keeping instructions of “when you come into the land”, or “when you cross over” saying that YHVH didn’t really mean that because the people kept Feasts outside of the land in the wilderness. This is not true and I cannot find any supporting scripture that would tell us so.

As well, who is the only group presiding over Feasts or for that matter, blowing a shofar to establish to the nation of Israel, the exact calendar day? All of Israel would need to be reconciled to the exact calendar of everyone else. This was the job of the presiding priests of the Levitical priesthood.

Hos 2:11 I will stop all her celebrations:
her yearly festivals, her New Moons,
her Sabbath days—all her appointed festivals.

YHVH did this by vomiting Israel from the land just as He warned He would.