Hi Luben,
Normally you can define the version in the SAPUI5 bootstrap by changing the SDK source. You can change the version in the url:
/sap/public/bc/ui5_ui5/1.26/resources/sap-ui-core.js
As you already mentioned, this is easy when to app runs standalone. But when it's started from the Fiori launchpad it will not go to the index.html, instead it will start from the component.js .
You can try to configure the SDK in the component.js metadata:
"sap.ui5": {
"_version": "1.1.0",
"rootView": {
"viewName": "com.view.main",
"type": "XML"
},
"dependencies": {
"minUI5Version": "1.30.0",
"libs": {
"sap.ui.core": {},
"sap.m": {},
"sap.ui.layout": {}
}
}
If that doesn't work, I would try to create a fiori tile based on the full url and not on a semantic object or launchpad configuration.
The last one will be the easiest way 
Kind regards,
Wouter