Whether "OM" appears in Veda Samhita?

"OM" appears in vAjasaneya mAdhyandina Sukla Yajur Veda SamhitA ii - 13
"OM pratishtha"

Prayer to Lord Vishnu for getting relief from Fever

vaidyo narayaNo hari:

apAmArjana stotram which is found in vishNu dharmottara purANam is a powerful prayer to Lord vishNu for maintaining good health. Those who are suffering from incurable fevers and diseases get effective cure by reciting this stotram which propitiates the Supreme mainly in the forms of varAha, narasimha, vAmana and vishNu.

WHETHER VEDIC SCRIPTURES CAN BE CHANGED TO ADAPT TO THE CHANGING TIMES?

The SAstras based on Sruti prescribe the ideal which one has to follow. Depending upon one's SraddhA ( faith) , deSa (place), kAla (time) and vartamAna (environment), religious practices are being done . Many a time we find that there is a big gap between what the Scriptures prescribe and what we do in practice.If we are unable to do as per Scriptures, we should feel sorry and offer our apologies to the devatAs(Deities) / Rishis/ pitrus (Manes) in a spirit of devotion and SraddhA.

Changing the Scriptures for the purpose of adaptability is not warranted nor is it permissible.

HOW TO INVOKE THE RAIN GOD?

As per Vedic tradition, there is a Yajña known as the KArIri Ishti for invoking the Rain God.

In addition we have:
1. VaruNa Mantra
2. RishyaSringa Slokas
3. Azhi Mazhai KaNNa ( of TiruppAvai)

These are very powerful in invoking the rains.
In Music , it is Amrutavarshini RAga.

CANONS OF PHILANTHROPY -- DANAM

As per TaittirIya Upanishad i-11, the canons of Philanthropy ( DAnam) are as follows:

1.Give with faith ( SraddhaYA deyam)
2.Do not give without faith. (aSraddhayA adeyam)
3.Give in plenty (SriyA deyam)
4.Give with modesty (hriyA deyam)
5.Give with sympathy ( bhiyA deyam)
6.Give with knowledge (samvidA deyam).

Whether Surgery is mentioned in the Vedas?

There is reference in the Vedas about Surgery and Dentistry.

Surgery
The Asvins fixed an equine head on Dadhyac. -- Brihadaranyaka Upanishad ii-5-17

Dentistry
Dentistry is mentioned in the Vedas. -- Aitareya Brahmana iii-6-3, Aitareya Brahmana ii-1-5

What are the Sutras of Shukla Yajur Veda?

SUtras pertaining to Sukla Yajur Veda:

1.PAraskara Grihya SUtra
2.KAtyAyana Srauta SUtra
3. KAtyAyana Sulba SUtra