Does the purchase of an Asset Slip on CouchTycoon make me a real shareholder of the venture?

No!

From the legal point of view: NO.
In fact: YES.
Mr. Idle holds the real shares in trust. PIPELINE ventures GmbH is the real shareholder of fully financed ventures. The fictional share called "Asset Slip" provides you the same rights like holding the real share. PROMISED!

See also: "What happens in the background?"