Pardon my lack of knowledge here. Any advice given is very much appreciated.

I loaded all 3 existing DLCs, so my maximum level is 45. My understanding is that upon reaching Level 45, you cannot gain any more experience, nor level up anymore.

1. Suppose that I have reached Level 45. Is there any disadvantage I would face? In other words, does any aspect of gaming in Fallout change? Can I still play Fallout: New Vegas as usual?

2. Assume that I gain another 200 XP, which is not included in my total experience points as I have reached Level 45. If I load the upcoming Lonesome Road DLC next week when it is released, will I gain back that 200 XP?

Thank you for the information.

Hitting the cap doesn't affect anything besides you no longer gaining xp you still play as normal completing quests, killing, exploring you just won't gain xp as you do so, and no you won't get any xp back if you hit the cap then do something to get xp like complete a quest or kill an enemy that xp is gone.

1. Yes, you can play as normal, you just won't gain exp any more.

2. No, any exp gained after hitting the cap is lost to the void.

This

The only major disadvantage is how some enemies will scale upwards. Most prominently, the OWB enemies and the HH enemies.

You also get more skills, which is a disadvantage to some people because it make your character less specialized and more generic.