The problem with discussions like these, and one that goes nearly always unspoken, is the problem of credibility.
When you deal with deceitful people, credibility matters.
The only time credibility does not matter is when you can independently verify what is being said, such as through a framework like Descartes Rules of Method, Logic, or some other epistemological method that is appropriate. Only then does credibility not matter because you can judge for yourself because it has appropriate rational backing.
When you are solely down to an entity that does not use such a framework; credibility matters.. and by extension reputation and past actions take on more importance than hollow words.
Microsoft, especially when it was run by Bill Gates, was never a credible entity. Today, it is run by accountants seeking to ride the knifes edge of what the market will bear, and in the process provide as little value as possible for creations that were made decades ago, ride the gravy train until it goes into the ocean.
While the people at Microsoft certainly have come up with some neat features and projects, when looking at these features/products and the history behind them (i.e. looking at Powershell), these were largely complete accidents, and/or significant internal politicking that could have gone either way because there was little to no profit in it. Their engineers used to be some of the best, but it matters little when the Accountants are in charge and see everything in a risk averse lens.
Once you lose credibility, it is almost impossible to get it back because you have to do so much more than it would have cost originally just to tip the scales back to even. Perception is sticky like that.
To get back to the core question, can we trust Microsoft with Open Source...
No you can't, not ever, because they are solely concerned with making profit and stealing whatever they can under the color of law. Open Source doesn't make profit.
The rise of open source is the natural outcome when barriers to entry in a market impose costs so high that you can't do business without those features, (excluding participants from that market, monopolistic ally), and open source is exploitive free labor that's been used to create an alternative to those products when no business can compete due to the simple mathematics of costs.
Even the big players can't compete with free for long without diversifying which is why FOSS is still around today. Volunteers, and some non-standard funding routes which aren't really all that viable at scale. This is also why Microsoft as an entity has been trying to poison pill Linux for awhile, they want to be the only game in town and read everything you write in Office; build a dossier, and sell it to the highest bidder. What could be more valuable then the thoughts of every single person using their product.
Of course they'll claim they aren't doing this (deceitfully), just like Google claims they don't wiretap and listen to surrounding endpoint devices they don't own to show you ads related to what you were talking about in your own home.