Hi,
1. No;
2. Depends: Case your function module works in a stateless way, it's easy to make them become OData Services.. Otherwise, re-write the code to be stateless.
3. No. jQuery is unnecessary dependency nowadays, usually people try to avoid using it for new projects.. however, UI5 has a direct dependency and is sort of built over jQuery.. so there is no way to avoid it now.
4. Case you can use SAP Portal (and J2EE Engine) opt for it instead of ABAP - the J2EE Engine has a much higher throughput than ABAP.. even working just on the Java stack (means you create an EJB, which calls a function modules, etc etc) will be faster than OData Service in ABAP.
5. Install the SDA's into a track, create some library with the UI5 version you want (in a way it embeds the resource servlet) and create a DC with the UI5 code.. it's the same as WDJ, there is no difference.
6. You don't - there is no tool for such since your estimation depends directly on resources. Create one proof of concept with perhaps one sample use case and track time.. probably your best choice.
7. Anyone with access to the client browser can impersonate and execute functions on behalf.. However, it's not usually the case.
Hope it helps,
Dan.