Alpine Route on May still snowing condition. So you need winter clothes.
But rest of the day are Spring season clothes.

Winter clothes eat alot luggage space. You still can't travel light liao 

Temperature at Alpine Route during May are 3 Celsius to 10 Celsius
You still able to see Snow Wall and do snow activities at Murodo

not snowing. You can see snow but not that kind of snow flake falling.

No need winter clothes. A spring coat alone is enough.

I tried peak season dates (late March) and can get RM1.5k. Oklar a bit savings from the RM1.7k i paid for this year
1. Allocate 1-day for Shin Hotaka Ropeway to be ideal. The bus journey alone is 1.5hr 1-way. There are a lot of onsen resorts nearby where you can spend a night.

2. Kurobe Alpine Route - stay the night in Toyama, so that you can get up early to start the route. Especially if you are planning to go around the opening date in mid-April (super peak period)

There's plenty to do in Odaiba other than just Gundam. You can visit FujiTV station, shop around Venus Fort/ DiverCity (the mall behind Gundam statue). There's also a Food Court in DiverCity - it's one of my fav eatery.

There's another Mall known as DECKS - check out the night view there. You can just Google "Rainbow Bridge Odaiba" and you'll see what I mean. Always find it's worthy to make a trip to Odaiba. You can arrange for something in the morning/ afternoon and make your way to Odaiba around 2 or 3pm onwards. It's about 1hr or so from Ginza.
Plenty. Like the above is some example.
Are you sure you're able to do so much in Takayama? Mind you, a lot of places close early and it gets dark around 4 or 5pm in autumn & winter. I barely managed to check out Takayama Shrine for an hour or so and then it got so dark, we head straight for dinner at 6pm.

Suggest you research how long it takes for you to get to Shin Hotaka Ropeway and back, as well as how long you'll spend there. Imo, the ropeway will easily take you 1 day without time for the rest. Better check the buses' operational hours as well - you wouldn't want to get stranded missing the last bus.
I always find "no time" is an excuse for an easy way out. Suggest you do some basic research by reading Page 1 Post #1. This is certainly not the place to look for a guide; as most if not all, would have made their own planning. Like Chaud mentioned, go with local ground tour or just walk around your area, or decide on the morning itself where you want to go.

Then will suggest you bold up the places where it's a must go, and the rest as back up if you have time. Most people plan for ideal situations, but that don't happen often I usually plan comfortably, with 1 or 2 place a backup if there's time.
It's up to you. So far there hasn't been exactly a bad review for WIFI router - they are quite stable. Some of the members have used Travel Recommend, HIS, Samurai WIFI etc. Check them out, and see if the price is competitive, as well as convenient for your collection and drop off.

It's quite impossible to visit all the Takayama attractions in a day based on your schedule. Imagine this

Earliest Hida Wide View express train from Nagoya is 7.45am. Reach Takayama around 10.15am. Next bus available is 10.40am and reach the base of Shin-Hotaka Ropeway at 12.25pm. Next available return bus schedule is 1.46pm (unlikely unless u rush) or 2.55pm. Arrived back in Takayama only by 3.41pm or 4.41pm.

It's best to allocate 1 day for Shin-Hotaka ropeway, and if you like, spend a night at various onsen along the route. Prices are very reasonable because there are not a lot of tourists visiting this area.

Bus schedule website for whole Takayama/Shirakawago/Shin-Hotaka/Kanazawa. Also check out the economical tickets/passes - https://www.nouhibus.co.jp/english/highwaybus/

Baggage courier services - http://www.global-yamato.com/en/hands-free...vel/facilities/

There's a Ta-Q-Bin shop just opposite of JR Takayama station. For Kanazawa, I can't remember if there's any baggage courier service at the station.

Just add more days then 

You are earning SGD after-all, while most of US have to suffer weaker Ringgit lel

EDIT
A few more links for you

Online reservation for bus route Takayama-Shirakawago-Kanazawa - https://www.japanbusonline.com/en/CourseSea...40002?afcd=MDI=

Value pack for Shin-Hotaka ropeway visit - https://www.nouhibus.co.jp/hida/bustrip/shi...a_spa_lunch-en/

If you intend to spend a night at Shirakawago - booking thru https://www.japaneseguesthouses.com/

What about the gassho-zukuri (triangle roof houses) in Shirakawa Go? It needs to be booked early as well because if it's last minute it's usually full. Your itinerary seems fine to me

Kamikochi? Not sure but ShirakawaGo is nearby Takayama. So your order of Takayama > ShirakawaGo is fine. Noticed you're planning to stay a night in ShirakawaGo, which prompted me to ask if you've booked the accommodation there haha because it's always fully booked especially winter.

I've not taken the route you planned so it's hard to comment. But one thing for sure, allow more time buffer for travelling because you may need to find the bus stop/ station, walk etc. In fact, there may be a change in timetable sometimes and we're not aware of it. So be sure to double check the bus schedule over there.

flight by ana?

Thanks. Going to add Shin-hotaka ropeway to my itinerary when I'm at Takayama. But the trip to Shin-hotaka ropeway will likely take at least a day? Considering the travel time there from Takayama would be around 1 1/2 hours and sundown would be around 5pm.

Not only to Japan, but to other destinations as well. Free seats? I couldn't find any for popular destinations like Japan, Aussie.
There are many onsens in Takayama depending on budget. But most are rather older type and many of them are located on the outskirts of Takayama, so most probably you will need a car to get there. The good ones are not cheap either. Try looking for those with halfboard as their dinner are usually very good.

Let me go back Home and dig out my 555 booklet record. It might have something.
Those Ryokan Onsen which open for public during daylight.  500 - 1200 Yen entrance fees

*Note: those onsen are traditional one, must naked 

It can be as cheap as free. Those public onsen. Only japanese know where it located.

Outskirt small onsen have a family room type onsen.

And even male/female same pool outdoor onsen with divider only. My experience. 

There are not cheap accomodation around Takayama areas, because it is already a very 'touristy' area. Its even crazier in Shirakawa Go. 

Private onsens? Sure there are. But the thing is price is not cheap.

Some onsens have private reservable bath that you can booked for a 1-hr slot for 1000yen++

My gf is not comfortable being naked in front of other ppl (except me of course  )