Now, I tried using an XML validator and it said everything was fine.
I’ve used this online validator and can confirm that validation works just fine, if you attach the BPMN schema inside an xsi:schemaLocation attribute. This is what I indicated earlier, this is what the validator tells you about. Of course your diagram is structural valid XML, that does not make it a valid BPMN 2.0 document.
Example diagram I used:
<?xml version="1.0" encoding="UTF-8"?>
<bpmn:definitions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:bpmn="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:di="http://www.omg.org/spec/DD/20100524/DI" xmlns:dc="http://www.omg.org/spec/DD/20100524/DC" xsi:schemaLocation="http://www.omg.org/spec/BPMN/20100524/MODEL http://www.omg.org/spec/BPMN/2.0/20100501/BPMN20.xsd http://www.omg.org/spec/DD/20100524/DC https://www.omg.org/spec/BPMN/20100501/DC.xsd http://www.omg.org/spec/BPMN/20100524/DI https://www.omg.org/spec/BPMN/20100501/BPMNDI.xsd http://www.omg.org/spec/DD/20100524/DI https://www.omg.org/spec/BPMN/20100501/DI.xsd" id="Definitions_1" targetNamespace="http://bpmn.io/schema/bpmn">
<bpmn:process id="Process_1" isExecutable="false">
<bpmn:startEvent id="StartEvent_1" />
<bpmn:task id="Task_1" />
</bpmn:process>
<bpmndi:BPMNDiagram id="BPMNDiagram_1">
<bpmndi:BPMNPlane id="BPMNPlane_1" bpmnElement="Process_1">
<bpmndi:BPMNShape id="_BPMNShape_StartEvent_2" bpmnElement="StartEvent_1">
<dc:Bounds x="173" y="102" width="36" height="36" />
</bpmndi:BPMNShape>
<bpmndi:BPMNShape id="Task_1_di" bpmnElement="Task_1">
<dc:Bounds x="353" y="80" width="100" height="80" />
</bpmndi:BPMNShape>
</bpmndi:BPMNPlane>
</bpmndi:BPMNDiagram>
</bpmn:definitions>
Copy the xsi:schemaLocation and xmlns:xsi attribute over to your files to get BPMN validation.
I already installed everything with the prepackage version so there is no error with the imports
Did you check if there is warnings with the imports? Using your diagram in another tool I see tons of warnings.