On the finance-page I see something called "risk capital" or similar (I play on German).
This "risk capital" is probably where it shows "investment" in english, first entry in ledger, yes?
That is where it shows if you have watched a video from Facebook to get $$.

How can I imprive Merchandise income? Got 2,68M fans and only 3.41 M income from merchandise. A bit few, for the middle of the season, isn't it?
Facilities like parking, eventually add to merchandise total by having most fans possible at matches, the Facilities Restaurant Gift Shop is where mechandise is sold, so having that as upgraded as you can helps.

How can I check whether my entry price is okay? I am in League stage 17, my stadium is fully upgraded and has space for 211.000 fans. Where do I see how many fans are inside the stadium so that I can optimize entry prices?

In your fixtures where you review a particular match the Spectators will be shown along with attendance. I tend to have my ticket price set to get full attendance, not concern for profit. If you have the price too high, or your team isn't doing well, attendance will fall. Start at a third of possible price and go up and stop when you still have good attendance.
Maximum capacity by level:
L1 - 800
L2 - 2,500
L3 - 6,000
L4 - 11,500
L5 - 19,000
L6 - 28,000
L7 - 40,000
L8 - 53,500
L9 - 69,000
L10 - 86,500
L11 - 106,000
L12 - 127,500
L13 - 151,000
L14 - 176,500
L15 - 204,000
L16 onwards - 260,000