Your posts covers a lot of ground, with several nuanced ideas.

First of all for material participation, the 100 days are not prorated for an activity that starts during the year. It is also not limited to the calendar year that the tax return covers.  It applies to the period of 365 days, from when the activity starts, even if that spans two calendar years.  The word "year" by itself does not mean "taxable year".  Unless, someone is participating more than your clients, it sounds like they would materially participate.

Meeting the material participation test, does not "cause the property to be taxed like a business".  In the case of a short term rental it merely means that the activity is not subject to passive activity loss limitations.  It is still not subject to self employment taxes unless "significant services" are provided

I googled around for people asserting that IRC 195 does not apply to rental activities, and was able to find a couple.  They made their determination based on the IRC 195 phrase "active trade or business" vs the IRC 469 phrase "passive activity" or a trade or business in which the taxpayer does not materially participate.

In my opinion, when IRC 195 uses the phrase active trade or business they are not using that to distinguish between a passive or non passive activity. Rather, they are using the word active to refer to a trade or business which has begun, and is not a "start up".  Therefore, IRC 195 applies to rental activities that are considered to be a trade or business.

As to whether a rental activity is considered to be a trade or business, this was a subject of broad conversation when IRC 199A came out, and I believe the case law on this subject sets a very low bar for the amount of involvement considered to be "regular and continuous" the necessary requirement to be considered a trade or business.

Consider this, have you ever applied the capital loss limitation of $3,000 to the sale of a rental property with a loss?  It is the same requirement, a IRC 1231 loss requires that the rental property be used in a trade or business otherwise the loss would be capped at $3,000 per IRC 1211.

In your situation, with roughly 6 weeks between purchase and rental as well as the low amount of actual expenses involved, I don't think I would apply the start up cost limits in this situation.  If it was a longer time frame, and more money involved I would apply IRC 195, and I would not capitalize them to basis in the property.

So in summary, after much consideration my opinion would be:

Yes, they materially participated

Yes, this is an active trade or business

No, I would not treat these as "start up" expense, just regular expenses.

If you are not comfortable treating them as regular expenses, then I would treat them as start up expenses.