Best way to process large/complex XML/schema ?

[snippsat]

Here e demo of using lxml with xPath.

from lxml import etree

root = etree.parse('W2Testfile.xml')
sub = root.xpath('//SubmissionId')[0]
print(sub.text)
for tag in root.xpath('//DependentRelationshipCd'):
    print(tag.text)

00000000000000002222
SON
HALF BROTHER
HALF BROTHER
STEPCHILD

We may have 45K to 50K xml elements to traverse this way.

Not that large would not cause any problems.

Is lxml work as DOM serial parsing ? How will address of pulling all this big XML into DOM?

lxml can operate in either mode,it just depends how your code uses it.
Can either parse the full file into a DOM, or use Sax callbacks to parse serially.

Are there any options in Python do parallel parsing like SAX (Java)?

Yes serval ways eg concurrent.futures

Try some stuff and see how it goes.