It’s my understanding banks have only two ways to issue Unwiderrufliches Zahlungsversprechen (construction is still in progress):
Is there any other option? For example, I transfer 3X worth of VT stocks/bonds to their management, or something like that?
Both points have to be given; signing the contract (commitment) + transferring needed cash to the bank (no assets = no guarantee by the bank).
I do not know any financing instution who accepts stocks/ETF as collateral since they can go theoretically go bust and the value is CHF 0 - even if you are a high net worth individual, the compliance is pretty strict regarding this topic. Therefore, if you want to work with a guarantee, then you have to guarantee the value.
Ergo: no, you have to sell you shares.
Yes, it is called a Lombard loan.
How many weeks do I have between making a reservation and the notarization meeting? The seller told me about 6 weeks, but I don’t see it anywhere written down, so what prevents them from scheduling the meeting in 10 days?
I want to see how much time I have to pick the best financing option (and transfer cash to a bank).
There is no rule, I see everything from two weeks to three months. Depends, how fast the seller/real estate agent wants to sell.
The following will preventing you to have the notarization in 10 days: the bank needs time to give you the offer. Let’s assume, you got it today. They will order the contracts internally, which takes at the moment three business days. Then one day lost by sending it by postal service. Reading, signing and sendig back leads to one more day lost.
Transferring cash as soon as the IBAN is ready + selling the securities takes probably three days because of settlement of securities. Issuing the promise of payment (only after the cash is transferred and the contract signed) takes another day.
If you have no buffer days (e.g. relationship manager is sick, document get lost by postal service, irrevocable promise of payments gets lost) 10 days is pretty tight.
If your business case is really, really, important and you come by to the branch to sign the contract and you have your assets transferred, then maybe 5 business days is possible. But the relationship manager, the credit officer and the internal staff will hate you for this ultra short term action 
Four weeks is the way to go (depending as well, if you use cash from 2nd pillar and pillar 3a).
Thanks. So, I can assume seller is not an asshole that will give me some unreasonable deadline to do all this?
Yeah, I would assume so.
No interest in making your life hard, he was in the same situation once.
Would Notary accept Bankgarantie instead of UZ?