(2) there is no reinvest, so the 2-transaction-way is the only way you have if you want to keep your performance data correct.
regarding the rounding: if you receive 90 shares at 1,1€ then i would use 90*1,1€ as the payout value. the lost dollar cuts into your performance and is not actualy something you get. But this depends on what you want to record in pp. The 100% correct transaction, or the correct performance.
(1) I thought you were talking about 100% unlisted assets. assets with no yahoo history but ISIN/Ticker can usualy be imported in other ways. Quellen für historische Kurse is a compilation of some ways. If you can find a table with a static URL, then you can use this instead of yahoo finance. if the asset is on alpha-vantage, then you can sign up there for a free account and use your id to get quotes.
the “.” and “,” problem in csv can be fixed beforehand with find&replace. not sure how to handle it within the automatic price import.