So, a scenario would be that my local restaurant accepts BTC and one dinner would cost me 0.0000001550 BTC and when I make payment it costs me 15,50 Satoshis. This must mean that there exists a potential for wallet services that make conversions between different BTC units of account - in a mainstream BTC scenario.
No need to "make conversions" The current wallet already does all it's internal accounting in terms of Satoshis. As the daily interactions move towards smaller units, the wallet interface will be configured to display in terms of these smaller units.
Btw - you seem knowledgable and informed:) Perhaps do you know whether the transaction time of BTC will ever go lower than 10 mins?
Transaction time is already much less than 10 minutes. Most transactions are relayed throughout the bitcoin network in a few seconds. Confirmations of transactions on the other hand will always require a block, and the protocol is designed to keep the average time between blocks close to 10 minutes. That actual time for any particular block is somewhat random. They can (and have) occur in a few seconds. They also can (and have) occur in over 90 minutes. This randomness and adjustment towards 10 minute averages will almost certainly not change.
And/or is there a potential for intermediary parties to make transaction happen instantenously on demand?
Yes, there is definitely potential for intermediary parties to provide instantaneous guarantees of transaction confirmation. I suspect that such intermediaries will come into existence prior to bitcoin being widely accepted in "mainstream" use.